Process for producing humic acid-derived conductive foams

ABSTRACT

A process for producing a humic acid (HA)-derived foam, comprising: (a) preparing a HA dispersion having multiple HA molecules and an optional blowing agent dispersed in a liquid medium having a blowing agent-to-HA weight ratio from 0/1.0 to 1.0/1.0; (b) dispensing and depositing the HA dispersion onto a surface of a supporting substrate to form a wet HA layer; (c) partially or completely removing liquid medium from the wet HA layer to form a dried HA layer; and (d) heat treating the dried HA layer at a first heat treatment temperature from 80° C. to 3,200° C. at a desired heating rate sufficient to induce volatile gas molecules from the non-carbon elements or to activate the blowing agent for producing the HA-derived foam.

FIELD OF THE INVENTION

The present invention relates generally to the field of carbon/graphitefoams and, more particularly, to a new form of conductive foam derivedfrom humic acid, devices containing such a humic acid-derived foam, andthe process for producing same.

BACKGROUND OF THE INVENTION

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube orcarbon nano-fiber (1-D nano graphitic material), graphene (2-D nanographitic material), and graphite (3-D graphitic material). Other thanfullerene, all these materials can be made into a foamed structure.

The carbon nano-tube (CNT) refers to a tubular structure grown with asingle wall or multi-wall. Carbon nano-tubes (CNTs) and carbonnano-fibers (CNFs) have a diameter on the order of a few nanometers to afew hundred nanometers. Their longitudinal, hollow structures impartunique mechanical, electrical and chemical properties to the material.The CNT or CNF is a one-dimensional nano carbon or 1-D nano graphitematerial. However, CNTs are difficult to produce and are extremelyexpensive. Further, CNTs are known to be difficult to disperse in asolvent or water and difficult to mix with other materials. Thesecharacteristics have severely limited their scope of application.

A single-layer graphene sheet is composed of carbon atoms occupying atwo-dimensional hexagonal lattice. Multi-layer graphene is a plateletcomposed of more than one graphene plane. Individual single-layergraphene sheets and multi-layer graphene platelets are hereincollectively called nano graphene platelets (NGPs) or graphenematerials. NGPs include pristine graphene (essentially 99% of carbonatoms), slightly oxidized graphene (<5% by weight of oxygen), andgraphene oxide (≥5% by weight of oxygen).

NGPs have been found to have a range of unusual physical, chemical, andmechanical properties. Our research group was the first to discovergraphene [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,”U.S. patent application Ser. No. 10/274,473, submitted on Oct. 21, 2002;now U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. The processes for producingNGPs and NGP nanocomposites were previously reviewed by us [Bor Z. Jangand A Zhamu, “Processing of Nano Graphene Platelets (NGPs) and NGPNanocomposites: A Review,” J. Materials Sci. 43 (2008) 5092-5101]. Fourmain prior-art approaches have been followed to produce NGPs. Theiradvantages and shortcomings are briefly summarized as follows:

Approach 1: Chemical Formation and Reduction of Graphene Oxide (GO)

The first approach (FIG. 1) entails treating natural graphite powderwith an intercalant and an oxidant (e.g., concentrated sulfuric acid andnitric acid, respectively) to obtain a graphite intercalation compound(GIC) or, actually, graphite oxide (GO). [William S. Hummers, Jr., etal., Preparation of Graphitic Oxide, Journal of the American ChemicalSociety, 1958, p. 1339.] Prior to intercalation or oxidation, graphitehas an inter-graphene plane spacing of approximately 0.335 nm(L_(d)=½d₀₀₂=0.335 nm). With an intercalation and oxidation treatment,the inter-graphene spacing is increased to a value typically greaterthan 0.6 nm. This is the first expansion stage experienced by thegraphite material during this chemical route. The obtained GIC or GO isthen subjected to further expansion (often referred to as exfoliation)using either a thermal shock exposure or a solution-based,ultrasonication-assisted graphene layer exfoliation approach.

In the thermal shock exposure approach, the GIC or GO is exposed to ahigh temperature (typically 800-1,050° C.) for a short period of time(typically 15 to 60 seconds) to exfoliate or expand the GIC or GO forthe formation of exfoliated or further expanded graphite, which istypically in the form of a “graphite worm” composed of graphite flakesthat are still interconnected with one another. This thermal shockprocedure can produce some separated graphite flakes or graphene sheets,but normally the majority of graphite flakes remain interconnected.Typically, the exfoliated graphite or graphite worm is then subjected toa flake separation treatment using air milling, mechanical shearing, orultrasonication in water. Hence, approach 1 basically entails threedistinct procedures: first expansion (oxidation or intercalation),further expansion (or “exfoliation”), and separation.

In the solution-based separation approach, the expanded or exfoliated GOpowder is dispersed in water or aqueous alcohol solution, which issubjected to ultrasonication. It is important to note that in theseprocesses, ultrasonification is used after intercalation and oxidationof graphite (i.e., after first expansion) and typically after thermalshock exposure of the resulting GIC or GO (after second expansion).Alternatively, the GO powder dispersed in water is subjected to an ionexchange or lengthy purification procedure in such a manner that therepulsive forces between ions residing in the inter-planar spacesovercome the inter-graphene van der Waals forces, resulting in graphenelayer separations.

There are several major problems associated with this conventionalchemical production process:

-   -   (1) The process requires the use of large quantities of several        undesirable chemicals, such as sulfuric acid, nitric acid, and        potassium permanganate or sodium chlorate.    -   (2) The chemical treatment process requires a long intercalation        and oxidation time, typically 5 hours to five days.    -   (3) Strong acids consume a significant amount of graphite during        this long intercalation or oxidation process by “eating their        way into the graphite” (converting graphite into carbon dioxide,        which is lost in the process). It is not unusual to lose 20-50%        by weight of the graphite material immersed in strong acids and        oxidizers.    -   (4) The thermal exfoliation requires a high temperature        (typically 800-1,200° C.) and, hence, is a highly        energy-intensive process.    -   (5) Both heat- and solution-induced exfoliation approaches        require a very tedious washing and purification step. For        instance, typically 2.5 kg of water is used to wash and recover        1 gram of GIC, producing huge quantities of waste water that        need to be properly treated.    -   (6) In both the heat- and solution-induced exfoliation        approaches, the resulting products are GO platelets that must        undergo a further chemical reduction treatment to reduce the        oxygen content. Typically even after reduction, the electrical        conductivity of GO platelets remains much lower than that of        pristine graphene. Furthermore, the reduction procedure often        involves the utilization of toxic chemicals, such as hydrazine.    -   (7) Furthermore, the quantity of intercalation solution retained        on the flakes after draining may range from 20 to 150 parts of        solution by weight per 100 parts by weight of graphite flakes        (pph) and more typically about 50 to 120 pph. During the        high-temperature exfoliation, the residual intercalate species        retained by the flakes decompose to produce various species of        sulfuric and nitrous compounds (e.g., NO_(x) and SO_(x)), which        are undesirable. The effluents require expensive remediation        procedures in order not to have an adverse environmental impact.        Approach 2: Direct Formation of Pristine Nano Graphene Platelets

In 2002, our research team succeeded in isolating single-layer andmulti-layer graphene sheets from partially carbonized or graphitizedpolymeric carbons, which were obtained from a polymer or pitch precursor[B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/274,473, submitted on Oct. 21, 2002; now U.S.Pat. No. 7,071,258 (Jul. 4, 2006)]. Mack, et al [“Chemical manufactureof nanostructured materials” U.S. Pat. No. 6,872,330 (Mar. 29, 2005)]developed a process that involved intercalating graphite with potassiummelt and contacting the resulting K-intercalated graphite with alcohol,producing violently exfoliated graphite containing NGPs. The processmust be carefully conducted in a vacuum or an extremely dry glove boxenvironment since pure alkali metals, such as potassium and sodium, areextremely sensitive to moisture and pose an explosion danger. Thisprocess is not amenable to the mass production of NGPs.

Approach 3: Epitaxial Growth and Chemical Vapor Deposition of NanoGraphene Sheets on Inorganic Crystal Surfaces

Small-scale production of ultra-thin graphene sheets on a substrate canbe obtained by thermal decomposition-based epitaxial growth and a laserdesorption-ionization technique. [Walt A. DeHeer, Claire Berger, PhillipN. First, “Patterned thin film graphite devices and method for makingsame” U.S. Pat. No. 7,327,000 B2 (Jun. 12, 2003)] Epitaxial films ofgraphite with only one or a few atomic layers are of technological andscientific significance due to their peculiar characteristics and greatpotential as a device substrate. However, these processes are notsuitable for mass production of isolated graphene sheets for compositematerials and energy storage applications.

Another process for producing graphene, in a thin film form (typically<2 nm in thickness), is the catalytic chemical vapor deposition process.This catalytic CVD involves catalytic decomposition of hydrocarbon gas(e.g. C₂H₄) on Ni or Cu surface to form single-layer or few-layergraphene. With Ni or Cu being the catalyst, carbon atoms obtained viadecomposition of hydrocarbon gas molecules at a temperature of800-1,000° C. are directly deposited onto Cu foil surface orprecipitated out to the surface of a Ni foil from a Ni-C solid solutionstate to form a sheet of single-layer or few-layer graphene (less than 5layers). The Ni- or Cu-catalyzed CVD process does not lend itself to thedeposition of more than 5 graphene planes (typically <2 nm) beyond whichthe underlying Ni or Cu layer can no longer provide any catalyticeffect. The CVD graphene films are extremely expensive.

Approach 4: The Bottom-Up Approach (Synthesis of Graphene from SmallMolecules)

Yang, et al. [“Two-dimensional Graphene Nano-ribbons,” J. Am. Chem. Soc.130 (2008) 4216-17] synthesized nano graphene sheets with lengths of upto 12 nm using a method that began with Suzuki-Miyaura coupling of1,4-diiodo-2,3,5,6-tetraphenyl-benzene with 4-bromophenylboronic acid.The resulting hexaphenylbenzene derivative was further derivatized andring-fused into small graphene sheets. This is a slow process that thusfar has produced very small graphene sheets.

Hence, an urgent need exists to have a new class of carbon nanomaterials that are comparable or superior to graphene in terms ofproperties, but can be produced more cost-effectively, faster, morescalable, and in a more environmentally benign manner. The productionprocess for such a new carbon nano material requires a reduced amount ofundesirable chemical (or elimination of these chemicals all together),shortened process time, less energy consumption, reduced or eliminatedeffluents of undesirable chemical species into the drainage (e.g.,sulfuric acid) or into the air (e.g., SO₂ and NO₂). Furthermore, oneshould be able to readily make this new nano material into a foamstructure that is relatively conductive thermally and electrically.

Generally speaking, a foam or foamed material is composed of pores (orcells) and pore walls (a solid material). The pores can beinterconnected to form an open-cell foam. As an example, graphene foamis composed of pores and pore walls that contain a graphene material.There are three major methods of producing graphene foams, which are alltedious, energy-intensive, and slow:

The first method is the hydrothermal reduction of graphene oxidehydrogel that typically involves sealing graphene oxide (GO) aqueoussuspension in a high-pressure autoclave and heating the GO suspensionunder a high pressure (tens or hundreds of atm) at a temperaturetypically in the range of 180-300° C. for an extended period of time(typically 12-36 hours). A useful reference for this method is givenhere: Y. Xu, et al. “Self-Assembled Graphene Hydrogel via a One-StepHydrothermal Process,” ACS Nano 2010, 4, 4324-4330. There are severalmajor issues associated with this method: (a) The high pressurerequirement makes it an impractical method for industrial-scaleproduction. For one thing, this process cannot be conducted on acontinuous basis. (b) It is difficult, if not impossible, to exercisecontrol over the pore size and the porosity level of the resultingporous structure. (c) There is no flexibility in terms of varying theshape and size of the resulting reduced graphene oxide (RGO) material(e.g. it cannot be made into a film shape). (d) The method involves theuse of an ultra-low concentration of GO suspended in water (e.g. 2mg/mL=2 g/L=2 kg/kL). With the removal of non-carbon elements (up to50%), one can only produce less than 2 kg of graphene material (RGO) per1000-liter suspension. Furthermore, it is practically impossible tooperate a 1000-liter reactor that has to withstand the conditions of ahigh temperature and a high pressure. Clearly, this is not a scalableprocess for mass production of porous graphene structures.

The second method is based on a template-assisted catalytic CVD process,which involves CVD deposition of graphene on a sacrificial template(e.g. Ni foam). The graphene material conforms to the shape anddimensions of the Ni foam structure. The Ni foam is then etched awayusing an etching agent, leaving behind a monolith of graphene skeletonthat is essentially an open-cell foam. A useful reference for thismethod is given here: Zongping Chen, et al., “Three-dimensional flexibleand conductive interconnected graphene networks grown by chemical vapourdeposition,” Nature Materials, 10 (June 2011) 424-428. There are severalproblems associated with such a process: (a) the catalytic CVD isintrinsically a very slow, highly energy-intensive, and expensiveprocess; (b) the etching agent is typically a highly undesirablechemical and the resulting Ni-containing etching solution is a source ofpollution. It is very difficult and expensive to recover or recycle thedissolved Ni metal from the etchant solution. (c) It is challenging tomaintain the shape and dimensions of the graphene foam without damagingthe cell walls when the Ni foam is being etched away. The resultinggraphene foam is typically very brittle and fragile. (d) The transportof the CVD precursor gas (e.g. hydrocarbon) into the interior of a metalfoam can be difficult, resulting in a non-uniform structure, sincecertain spots inside the sacrificial metal foam may not be accessible tothe CVD precursor gas.

The third method of producing graphene foam also makes use of asacrificial material (e.g. colloidal polystyrene particles, PS) that iscoated with graphene oxide sheets using a self-assembly approach. Forinstance, Choi, et al. prepared chemically modified graphene (CMG) paperin two steps: fabrication of free-standing PS/CMG films by vacuumfiltration of a mixed aqueous colloidal suspension of CMG and PS (2.0 μmPS spheres), followed by removal of PS beads to generate 3D macro-pores[B. G. Choi, et al., “3D Macroporous Graphene Frameworks forSupercapacitors with High Energy and Power Densities,” ACS Nano, 6(2012) 4020-4028.]. Choi, et al. fabricated well-ordered free-standingPS/CMG paper by filtration, which began with separately preparing anegatively charged CMG colloidal and a positively charged PS suspension.A mixture of CMG colloidal and PS suspension was dispersed in solutionunder controlled pH (=2), where the two compounds had the same surfacecharges (zeta potential values of +13±2.4 mV for CMG and +68±5.6 mV forPS). When the pH was raised to 6, CMGs (zeta potential=−29±3.7 mV) andPS spheres (zeta potential=+51±2.5 mV) were assembled due to theelectrostatic interactions and hydrophobic characteristics between them,and these were subsequently integrated into PS/CMG composite paperthrough a filtering process. This method also has several shortcomings:(a) This method requires very tedious chemical treatments of bothgraphene oxide and PS particles. (b) The removal of PS by toluene alsoleads to weakened macro-porous structures. (c) Toluene is a highlyregulated chemical and must be treated with extreme caution. (d) Thepore sizes are typically excessively big (e.g. several μm), too big formany useful applications.

The above discussion clearly indicates that every prior art method orprocess for producing graphene and graphene foams has majordeficiencies. Thus, it is an object of the present invention to providea new class of foam material that is thermally and electricallyconducting and mechanically robust and to provide a cost-effectivemethod of producing this class of foam.

Humic acid (HA) is an organic matter commonly found in soil and can beextracted from the soil using a base (e.g. KOH). HA can also beextracted, with a high yield, from a type of coal called leonardite,which is a highly oxidized version of lignite coal. HA extracted fromleonardite contains a number of oxygenated groups (e.g. carboxyl groups)located around the edges of the graphene-like molecular center (SP² coreof hexagonal carbon structure). This material is slightly similar tographene oxide (GO) which is produced by strong acid oxidation ofnatural graphite. HA has a typical oxygen content of 5% to 42% by weight(other major elements being carbon and hydrogen). HA, after chemical orthermal reduction, has an oxygen content of 0.01% to 5% by weight. Forclaim definition purposes in the instant application, humic acid (HA)refers to the entire oxygen content range, from 0.01% to 42% by weight.The reduced humic acid (RHA) is a special type of HA that has an oxygencontent of 0.01% to 5% by weight.

The present invention is directed at a new class of graphene-like 2Dmaterials (i.e. humic acid) that surprisingly can be converted into afoamed structure of high structural integrity. Thus, another object isto provide a cost-effective process for producing such a nano carbonfoam (specifically, humic acid-derived foam) in large quantities. Thisprocess does not involve the use of an environmentally unfriendlychemical. This method enables the flexible design and control of theporosity level and pore sizes.

It is another object of the present invention to provide a humicacid-derived foam that exhibits a thermal conductivity, electricalconductivity, elastic modulus, and/or strength comparable to or greaterthan those of the conventional graphite foams, carbon foams, or graphenefoams. Yet another object of the present invention is to provide a humicacid-derived foam that preferably has a meso-scaled pore size range(2-50 nm).

Another object of the present invention is to provide products (e.g.devices) that contain a humic acid-derived foam of the present inventionand methods of operating these products.

SUMMARY OF THE INVENTION

The present invention provides a humic acid-derived foam composed ofmultiple pores and pore walls, wherein the pore walls containsingle-layer or few-layer humic acid-derived hexagonal carbon sheets andthe few-layer hexagonal carbon sheets have 2-10 stacked hexagonal carbonatomic planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to 0.60nm (preferably no greater than 0.40 nm) as measured by X-raydiffraction. The single-layer or few-layer hexagonal carbon sheetscontain 0.01% to 25% by weight of non-carbon elements. The humic acid(HA) is selected from oxidized humic acid, reduced humic acid,fluorinated humic acid, chlorinated humic acid, brominated humic acid,iodized humic acid, hydrogenated humic acid, nitrogenated humic acid,doped humic acid, chemically functionalized humic acid, or a combinationthereof.

The humic acid-derived foam herein invented can be divided into threetypes: (a) humic acid (HA) foams that contain at least 10% by weight(typically from 10% to 42% by weight and most typically from 10% to 25%)of non-carbon elements that can be used for a broad array ofapplications (wherein the original humic acid molecules remainsubstantially the same, but some chemical linking between HA moleculeshas occurred); (b) a chemically merged and reduced humic acid-based foamwherein extensive linking and merging between original HA molecules hasoccurred to form incipient graphene-like hexagonal carbon sheetsconstituting pore walls, resulting in evolution of chemical speciescontaining non-carbon elements originally attached to HA molecules(hence, non-carbon element content reduced to generally between 2% and10% by wt.); and (c) humic acid-derived graphitic foam that containsessentially all carbon only (<2% by weight of non-carbon content,preferably <1%, and further preferably <0.1%), wherein the pore wallscontain single-layer or few-layer (2-10) graphene-like sheets that arehexagonal carbon atomic planes.

Preferably and typically, the HA-derived foam has a density from 0.005to 1.7 g/cm³, a specific surface area from 50 to 3,200 m²/g, a thermalconductivity of at least 100 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 500 S/cm per unit of specificgravity. More typically, the humic acid-derived foam has a density from0.01 to 1.5 g/cm³ or an average pore size from 2 nm to 50 nm. In anembodiment, the foam has a specific surface area from 200 to 2,000 m²/gor a density from 0.1 to 1.3 g/cm³.

Typically, if the HA-derived foam is produced from a process that doesnot contain a heat treatment temperature (HTT) higher than 300° C., thefoam has a content of non-carbon elements in the range of 10% to 42% byweight. The pore walls can still contain identifiable humic acidmolecules that are sheet-like hexagonal carbon atomic structures. Thenon-carbon elements can include an element selected from oxygen,fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, or boron. In aspecific embodiment, the pore walls contain fluorinated humic acid andthe foam contains a fluorine content from 0.01% to 25% by weight. Inanother embodiment, the foam contains an oxygen content from 0.01% to25% by weight.

With a HTT higher than 300° C., neighboring HA molecules that areclosely packed and well-aligned can be chemically linked together toform multi-ring aromatic structures that resemble incipientgraphene-like hexagonal carbon atomic structures. As heat treatmentcontinues, these highly aromatic molecules are merged together in anedge-to-edge manner to increase the length and width of graphene-likehexagonal planes and, concurrently, several hexagonal carbon planes canbe stacked together to form multi-layer carbon atomic structures,similar to few-layer graphene structures. The inter-planar spacing istypically reduced to <<0.60 nm, more typically <0.40 nm. If the HTT isfrom 300° C. up to 1,500° C., one typically produces chemically mergedand reduced humic acid-based foam, wherein extensive linking and mergingbetween original HA molecules has occurred to form incipientgraphene-like hexagonal carbon sheets that constitute pore walls. Thenon-carbon content in the foam is typically reduced to from 2% to 10%.

If the HTT is from 1,500° C. to 3,200° C. and the foam can becomeessentially a graphitic foam wherein the pore walls contain single-layeror few-layer graphene-like hexagonal carbon planes and the non-carboncontent is reduced to less than 2% by wt.

In a preferred embodiment, the foam is made into a continuous-lengthroll sheet form (a roll of a continuous foam sheet) having a thicknessno greater than 200 μm and a length of at least 1 meter long, preferablyat least 2 meters, further preferably at least 10 meters, and mostpreferably at least 100 meters. This sheet roll is produced by aroll-to-roll process. There has been no prior art HA-derivedgraphene-like foam that is made into a sheet roll form.

In a preferred embodiment, the HA-derived foam has an oxygen content ornon-carbon content less than 1% by weight, and the pore walls havestacked graphene-like planes having an inter-planar spacing less than0.35 nm, a thermal conductivity of at least 200 W/mK per unit ofspecific gravity, and/or an electrical conductivity no less than 1,000S/cm per unit of specific gravity.

In a further preferred embodiment, the HA-derived foam has an oxygencontent or non-carbon content less than 0.1% by weight and said porewalls contain stacked graphene-like hexagonal carbon atomic planeshaving an inter-planar spacing less than 0.34 nm, a thermal conductivityof at least 250 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 1,500 S/cm per unit of specific gravity.

In yet another preferred embodiment, the graphene foam has an oxygencontent or non-carbon content no greater than 0.01% by weight and saidpore walls contain stacked graphene-like planes having an inter-graphenespacing less than 0.336 nm, a mosaic spread value no greater than 0.7, athermal conductivity of at least 300 W/mK per unit of specific gravity,and/or an electrical conductivity no less than 2,000 S/cm per unit ofspecific gravity.

In still another preferred embodiment, the graphene foam has pore wallscontaining stacked graphene-like atomic planes having an inter-planarspacing less than 0.336 nm, a mosaic spread value no greater than 0.4, athermal conductivity greater than 400 W/mK per unit of specific gravity,and/or an electrical conductivity greater than 3,000 S/cm per unit ofspecific gravity.

In a preferred embodiment, the pore walls contain stacked graphene-likehexagonal carbon atomic planes having an inter-planar spacing less than0.337 nm and a mosaic spread value less than 1.0. In a preferredembodiment, the foam exhibits a degree of graphitization no less than80% (preferably no less than 90%) and/or a mosaic spread value less than0.4. In a preferred embodiment, the pore walls contain a 3D network ofinterconnected graphene-like hexagonal carbon atomic planes.

In a preferred embodiment, the solid foam contains meso-scaled poreshaving a pore size from 2 nm to 50 nm. The solid foam can also be madeto contain micron-scaled pores (1-500 μn).

The presently invented HA-derived foam may be produced by a processcomprising: (a) preparing a humic acid dispersion having multiple humicacid molecules or sheets dispersed in a liquid medium, wherein the humicacid is selected from oxidized humic acid, reduced humic acid,fluorinated humic acid, chlorinated humic acid, brominated humic acid,iodized humic acid, hydrogenated humic acid, nitrogenated humic acid,doped humic acid, chemically functionalized humic acid, or a combinationthereof and wherein the dispersion contains an optional blowing agenthaving a blowing agent-to-humic acid weight ratio from 0/1.0 to 1.0/1.0;(b) dispensing and depositing the graphene dispersion onto a surface ofa supporting substrate (e.g. plastic film, rubber sheet, metal foil,glass sheet, paper sheet, etc.) to form a wet layer of humic acid; (c)partially or completely removing the liquid medium from the wet layer ofhumic acid to form a dried layer of humic acid; and (d) heat treatingthe dried layer of humic acid at a first heat treatment temperature from80° C. to 3,200° C. at a desired heating rate sufficient to induceformation and releasing of volatile gas molecules from the non-carbonelements (e.g. O, H, N, B, F, Cl, Br, I, etc.) or to activate theblowing agent for producing humic acid-derived foam. Preferably, thedispensing and depositing procedure includes subjecting the humic aciddispersion to an orientation-inducing stress.

This optional blowing agent is not required if the HA material has acontent of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.) noless than 5% by weight (preferably no less than 10%, further preferablyno less than 20%, even more preferably no less than 30%). The subsequenthigh temperature treatment serves to remove a majority of thesenon-carbon elements from the edges of HA molecules, generating volatilegas species that produce pores or cells in the solid foam structure. Inother words, quite surprisingly, these non-carbon elements play the roleof a blowing agent. Hence, an externally added blowing agent is optional(not required). However, the use of a blowing agent can provide addedflexibility in regulating or adjusting the porosity level and pore sizesfor a desired application. The blowing agent is typically required ifthe non-carbon element content in the humic acid is less than 5%.

The blowing agent can be a physical blowing agent, a chemical blowingagent, a mixture thereof, a dissolution-and-leaching agent, or amechanically introduced blowing agent.

The process may further include a step of heat-treating the solid foamat a second heat treatment temperature higher than the first heattreatment temperature for a length of time sufficient for obtaining agraphene-like foam wherein the pore walls contain stacked hexagonalcarbon atomic planes having an inter-planar spacing d₀₀₂ from 0.3354 nmto 0.40 nm and a content of non-carbon elements less than 5% by weight(typically from 0.001% to 2%). When the resulting non-carbon elementcontent is from 0.1% to 2.0%, the inter-plane spacing d₀₀₂ is typicallyfrom 0.337 nm to 0.40 nm.

If the original HA molecules in the dispersion contains a non-carbonelement content higher than 5% by weight, the hexagonal carbon atomicplanes in the solid foam (after the heat treatment) contain structuraldefects that are induced during the step (d) of heat treating. Theliquid medium can be simply water and/or an alcohol, which isenvironmentally benign.

In a preferred embodiment, the process is a roll-to-roll process whereinsteps (b) and (c) include feeding the supporting substrate from a feederroller to a deposition zone, continuously or intermittently depositingthe HA dispersion onto a surface of the supporting substrate to form thewet layer of HA material thereon, drying the wet layer of HA material toform the dried layer of HA material, and collecting the dried layer ofHA material deposited on the supporting substrate on a collector roller.Such a roll-to-roll or reel-to-reel process is a truly industrial-scale,massive manufacturing process that can be automated.

In one embodiment, the first heat treatment temperature is from 100° C.to 1,500° C. In another embodiment, the second heat treatmenttemperature includes at least a temperature selected from (A) 300-1,500°C., (B) 1,500-2,100° C., and/or (C) 2,100-3,200° C. In a specificembodiment, the second heat treatment temperature includes a temperaturein the range of 300-1,500° C. for at least 1 hour and then a temperaturein the range of 1,500-3,200° C. for at least 1 hour.

There are several surprising results of conducting first and/or secondheat treatments to the dried HA layer, and different heat treatmenttemperature ranges enable us to achieve different purposes, such as (a)removal of non-carbon elements from the HA material (e.g. thermalreduction of fluorinated humic acid to obtain reduced humic acid) whichgenerate volatile gases to produce pores or cells in the HA foam, (b)activation of the chemical or physical blowing agent to produce pores orcells, (c) chemical linking or merging of humic acid molecules intohighly aromatic molecules and edge-to-edge merging of aromatic ringstructures or hexagonal carbon planes to significantly increase thelateral dimensions (length and width) of graphene-like hexagonal carbonsheets in the foam walls (solid portion of the foam), (d) healing ofdefects naturally existing in HA or created during fluorination,oxidation, or nitrogenation of humic acid molecules, and (e)re-organization and perfection of graphitic domains or graphitecrystals. These different purposes or functions are achieved todifferent extents within different temperature ranges. The non-carbonelements typically include an element selected from oxygen, fluorine,chlorine, bromine, iodine, nitrogen, hydrogen, or boron. Quitesurprisingly, even under low-temperature foaming conditions,heat-treating induces chemical linking, merging, or chemical bondingbetween sheet-like HA molecules, often in an edge-to-edge manner (somein face-to-face manner).

In one embodiment, the HA-derived foam has a specific surface area from200 to 2,000 m²/g. In one embodiment, the solid foam has a density from0.1 to 1.5 g/cm³. In an embodiment, step (d) of heat treating the layerof HA material at a first heat treatment temperature is conducted undera compressive stress. In another embodiment, the process comprises acompression step to reduce a thickness, pore size, or porosity level ofthe film of HA-derived foam. In some applications, the foam has athickness no greater than 200 μm.

In an embodiment, the HA dispersion has at least 5% by weight of HAdispersed in the liquid medium to form a liquid crystal phase. In anembodiment, the first heat treatment temperature contains a temperaturein the range of 80° C.-300° C. and, as a result, the HA foam has anoxygen content or non-carbon element content less than 5%, and the porewalls have an inter-planar spacing less than 0.40 nm, a thermalconductivity of at least 150 W/mK (more typically at least 200 W/mk) perunit of specific gravity, and/or an electrical conductivity no less than1,000 S/cm per unit of specific gravity.

In a preferred embodiment, the first and/or second heat treatmenttemperature contains a temperature in the range of 300° C.-1,500° C.and, as a result, the HA-derived foam has an oxygen content ornon-carbon content less than 2%, and the pore walls have an inter-planarspacing less than 0.35 nm, a thermal conductivity of at least 250 W/mKper unit of specific gravity, and/or an electrical conductivity no lessthan 1,500 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains atemperature in the range of 1,500° C.- 2,100° C., the HA-derived foamhas an oxygen content or non-carbon content less than 1% and pore wallshave an inter-graphene spacing less than 0.34 nm, a thermal conductivityof at least 300 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 3,000 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains atemperature greater than 2,100° C., the HA-derived foam has an oxygencontent or non-carbon content no greater than 0.1% and pore walls havean inter-planar spacing less than 0.336 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 350 W/mK per unitof specific gravity, and/or an electrical conductivity no less than3,500 S/cm per unit of specific gravity.

If the first and/or second heat treatment temperature contains atemperature no less than 2,500° C., the HA-derived foam has pore wallscontaining stacked graphene-like hexagonal carbon planes having aninter-planar spacing less than 0.336 nm, a mosaic spread value nogreater than 0.4, and a thermal conductivity greater than 400 W/mK perunit of specific gravity, and/or an electrical conductivity greater than4,000 S/cm per unit of specific gravity.

In one embodiment, the pore walls contain stacked graphene-likehexagonal carbon planes having an inter-planar spacing less than 0.337nm and a mosaic spread value less than 1.0. In another embodiment, thesolid wall portion of the HA-derived foam exhibits a degree ofgraphitization no less than 80% and/or a mosaic spread value less than0.4. In yet another embodiment, the solid wall portion of the HA-derivedfoam exhibits a degree of graphitization no less than 90% and/or amosaic spread value no greater than 0.4.

Typically, after a heat treatment at a HTT higher than 2,500° C., thepore walls in the HA-derived graphitic foam contain a 3D network ofinterconnected hexagonal carbon atomic planes that areelectron-conducting pathways. The cell walls contain graphitic domainsor graphite crystals having a lateral dimension (L_(a), length or width)no less than 20 nm, more typically and preferably no less than 40 nm,still more typically and preferably no less than 100 nm, still moretypically and preferably no less than 500 nm, often greater than 1 μm,and sometimes greater than 10 μm. The graphitic domains typically have athickness from 1 nm to 20 nm, more typically from 1 nm to 10 nm, andfurther more typically from 1 nm to 4 nm.

Preferably, the HA-derived foam contains meso-scaled pores having a poresize from 2 nm to 50 nm (preferably 2 nm to 25 nm).

In a preferred embodiment, the present invention provides a roll-to-rollprocess for producing a solid HA foam or HA-derived foam composed ofmultiple pores and pore walls The process comprises: (a) preparing ahumic acid dispersion having multiple humic acid molecules or sheetsdispersed in a liquid medium, wherein the humic acid is selected fromoxidized humic acid, reduced humic acid, fluorinated humic acid,chlorinated humic acid, brominated humic acid, iodized humic acid,hydrogenated humic acid, nitrogenated humic acid, doped humic acid,chemically functionalized humic acid, or a combination thereof andwherein the dispersion contains an optional blowing agent having ablowing agent-to-humic acid weight ratio from 0/1.0 to 1.0/1.0; (b)continuously or intermittently dispensing and depositing the HAdispersion onto a surface of a supporting substrate to form a wet layerof HA material, wherein the supporting substrate is a continuous thinfilm supplied from a feeder roller and collected on a collector roller;(c) partially or completely removing the liquid medium from the wetlayer of humic acid to form a dried layer of humic acid in a heatingzone or multiple heating zones; and (d) heat treating the dried layer ofhumic acid in one of these heating zones containing a heatingtemperature from 80° C. to 500° C. at a desired heating rate sufficientto activate the blowing agent for producing the humic acid-derived foamhaving a density from 0.01 to 1.7 g/cm³ or a specific surface area from50 to 3,000 m²/g. In this process, heat treatments occur in situ duringthe roll-to-roll procedure. This is a highly cost-effective processamenable to mass production of HA-derived graphitic foam sheets that arewrapped around on a roller for ease of shipping and handling and,subsequently, ease of cutting and slitting.

The orientation-inducing stress may be a shear stress. As an example,the shear stress can be encountered in a situation as simple as a“doctor's blade” that guides the spreading of HA dispersion over aplastic or glass surface with a sufficiently high shear rate during amanual casting process. As another example, an effectiveorientation-inducing stress is created in an automated roll-to-rollcoating process in which a “knife-on-roll” configuration dispenses thegraphene dispersion over a moving solid substrate, such as a plasticfilm. The relative motion between this moving film and the coating knifeacts to effect orientation of graphene sheets along the shear stressdirection. Comma coating and slot-die coating are particularly effectivemethods for this function.

This orientation-inducing stress is a critically important step in theproduction of the presently invented HA-derived foams due to thesurprising observation that the shear stress enables the HA molecules orsheets to align along a particular direction (e.g. X-direction orlength-direction) to produce preferred orientations and facilitatecontacts between HA molecules or sheets along foam walls. Furthersurprisingly, these preferred orientations and improved HA-to-HAcontacts facilitate chemical merging or linking between HA molecules orsheets during the subsequent heat treatment of the dried HA layer. Suchpreferred orientations and improved contacts are essential to theeventual attainment of exceptionally high thermal conductivity,electrical conductivity, elastic modulus, and mechanical strength of theresulting HA-derived foam. In general, these great properties could notbe obtained without such a shear stress-induced orientation control.

The present invention also provides an oil-removing or oil-separatingdevice containing the humic acid-derived foam as an oil-absorbingelement. Also provided is a solvent-removing or solvent-separatingdevice containing the humic acid-derived foam of as a solvent-absorbingor solvent-separating element.

The invention also provides a method to separate oil from water. Themethod comprises the steps of: (a) providing an oil-absorbing elementcomprising the integral humic acid-derived foam; (b) contacting anoil-water mixture with the element, which absorbs the oil from themixture; (c) retreating the element from the mixture and extracting theoil from the element; and (d) reusing the element.

Additionally, the invention provides a method to separate an organicsolvent from a solvent-water mixture or from a multiple-solvent mixture.The method comprises the steps of: (a) providing an organicsolvent-absorbing or solvent-separating element comprising the integralhumic acid-derived foam; (b) bringing the element in contact with anorganic solvent-water mixture or a multiple-solvent mixture containing afirst solvent and at least a second solvent; (c) allowing the element toabsorb the organic solvent from the mixture or separate the firstsolvent from the at least second solvent; (d) retreating the elementfrom the mixture and extracting the organic solvent or first solventfrom the element; and (e) reusing the element.

Also provided is a thermal management device containing the humicacid-derived foam as a heat spreading or heat dissipating element. Thethermal management device may contain a device selected from a heatexchanger, heat sink, heat pipe, high-conductivity insert, conductiveplate between a heat sink and a heat source, heat-spreading component,heat-dissipating component, thermal interface medium, or thermoelectricor Peltier cooling device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic drawing illustrating the processes for producinggraphene sheets from natural graphite particles.

FIG. 2 A possible mechanism of chemical linking and merging betweenhumic acid molecules and between “linked HA molecules.” Two or threeoriginal HA molecules can get chemically linked together to form longeror wider HA molecules, called “linked HA molecules”. Multiple “linked HAmolecules” can be merged to form graphene-like hexagonal carbon atomicplanes.

FIG. 3(A) Thermal conductivity values vs. specific gravity of theHA-derived foam produced by the presently invented process, meso-phasepitch-derived graphite foam, and Ni foam-template assisted CVD graphenefoam;

FIG. 3(B) Thermal conductivity values of the HA-derived foam,sacrificial plastic bead-templated

GO foam, and the hydrothermally reduced GO graphene foam.

FIG. 4 Electrical conductivity data from the HA-derived foam produced bythe presently invented process and the hydrothermally reduced GOgraphene foam.

FIG. 5 Thermal conductivity values of the foam samples, derived from HAand fluorinated HA, plotted as a function of the specific gravity.

FIG. 6 Thermal conductivity values of foam samples derived from HA andpristine graphene as a function of the final (maximum) heat treatmenttemperature.

FIG. 7 The amount of oil absorbed per gram of HA-derived foam is plottedas a function of the oxygen content in the foam having a porosity levelof approximately 98% (oil separation from oil-water mixture).

FIG. 8 The amount of oil absorbed per gram of integral HA-derived foam,plotted as a function of the porosity level (given the same oxygencontent).

FIG. 9 The amount of chloroform absorbed out of a chloroform-watermixture, plotted as a function of the degree of fluorination.

FIG. 10 Schematic of heat sink structures (2 examples).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Humic acid (HA) is an organic matter commonly found in soil and can beextracted from the soil using a base (e.g. KOH). HA can also beextracted from a type of coal called leonardite, which is a highlyoxidized version of lignite coal. HA extracted from leonardite containsa number of oxygenated groups (e.g. carboxyl groups) located around theedges of the graphene-like molecular center (SP² core of hexagonalcarbon structure). This material is slightly similar to graphene oxide(GO) which is produced by strong acid oxidation of natural graphite. HAhas a typical oxygen content of 5% to 42% by weight (other majorelements being carbon, hydrogen, and nitrogen). An example of themolecular structure for humic acid, having a variety of componentsincluding quinone, phenol, catechol and sugar moieties, is given inScheme 1 below (source: Stevenson H. J. “Humus Chemistry: Genesis,Composition, Reactions,” John Wiley & Sons, New York 1994).

Non-aqueous solvents for humic acid include polyethylene glycol,ethylene glycol, propylene glycol, an alcohol, a sugar alcohol, apolyglycerol, a glycol ether, an amine based solvent, an amide basedsolvent, an alkylene carbonate, an organic acid, or an inorganic acid.

The present invention provides a humic acid-derived foam composed ofmultiple pores and pore walls and a process for producing same. Thepores in the foam are formed during or after sheet-like humic acidmolecules are (1) chemically linked/merged together (edge-to-edge and/orface-to-face) typically at a temperature from 100 to 1,500° C. and/or(2) organized into larger graphite crystals or domains (herein referredto as graphitization) along the pore walls at a high temperature(typically >2,100° C. and more typically >2,500° C.).

The invention also provides a production process comprising: (a)preparing a humic acid dispersion having multiple humic acid moleculesor sheets dispersed in a liquid medium, wherein the humic acid isselected from oxidized humic acid, reduced humic acid, fluorinated humicacid, chlorinated humic acid, brominated humic acid, iodized humic acid,hydrogenated humic acid, nitrogenated humic acid, doped humic acid,chemically functionalized humic acid, or a combination thereof andwherein the dispersion contains an optional blowing agent having ablowing agent-to-humic acid weight ratio from 0/1.0 to 1.0/1.0; (b)dispensing and depositing the graphene dispersion onto a surface of asupporting substrate (e.g. plastic film, rubber sheet, metal foil, glasssheet, paper sheet, etc.) to form a wet layer of humic acid; (c)partially or completely removing the liquid medium from the wet layer ofhumic acid to form a dried layer of humic acid; and (d) heat treatingthe dried layer of humic acid at a first heat treatment temperature from80° C. to 3,200° C. at a desired heating rate sufficient to inducevolatile gas molecules from the non-carbon elements (e.g. O, H, N, B, F,Cl, Br, I, etc.) or to activate the blowing agent for producing humicacid-derived foam. Preferably, the dispensing and depositing procedureincludes subjecting the humic acid dispersion to an orientation-inducingstress. These non-carbon elements, when being removed via heat-induceddecomposition, produces volatile gases that act as a foaming agent orblowing agent.

The resulting humic acid-derived foam typically has a density from 0.005to 1.7 g/cm³ (more typically from 0.01 to 1.5 g/cm³, and even moretypically from 0.1 to 1.0 g/cm³, and most typically from 0.2 to 0.75g/cm³), or a specific surface area from 50 to 3,000 m²/g (more typicallyfrom 200 to 2,000 m²/g, and most typically from 500 to 1,500 m²/g).

A blowing agent or foaming agent is a substance which is capable ofproducing a cellular or foamed structure via a foaming process in avariety of materials that undergo hardening or phase transition, such aspolymers (plastics and rubbers), glass, and metals. They are typicallyapplied when the material being foamed is in a liquid state. It has notbeen previously known that a blowing agent can be used to create afoamed material while in a solid state. More significantly, it has notbeen previously taught or hinted that an aggregate of humic acidmolecules can be converted into a graphene-like foam via a blowingagent. The cellular structure in a matrix is typically created for thepurpose of reducing density, increasing thermal resistance and acousticinsulation, while increasing the thickness and relative stiffness of theoriginal polymer.

Blowing agents or related foaming mechanisms to create pores or cells(bubbles) in a matrix for producing a foamed or cellular material, canbe classified into the following groups:

-   -   (a) Physical blowing agents: e.g. hydrocarbons (e.g. pentane,        isopentane, cyclopentane), chlorofluorocarbons (CFCs),        hydrochlorofluorocarbons (HCFCs), and liquid CO₂. The        bubble/foam-producing process is endothermic, i.e. it needs heat        (e.g. from a melt process or the chemical exotherm due to        cross-linking), to volatize a liquid blowing agent.    -   (b) Chemical blowing agents: e.g. isocyanate, azo-, hydrazine        and other nitrogen-based materials (for thermoplastic and        elastomeric foams), sodium bicarbonate (e.g. baking soda, used        in thermoplastic foams). Here gaseous products and other        by-products are formed by a chemical reaction, promoted by        process or a reacting polymer's exothermic heat. Since the        blowing reaction involves forming low molecular weight compounds        that act as the blowing gas, additional exothermic heat is also        released. Powdered titanium hydride is used as a foaming agent        in the production of metal foams, as it decomposes to form        titanium and hydrogen gas at elevated temperatures.        Zirconium (II) hydride is used for the same purpose. Once formed        the low molecular weight compounds will never revert to the        original blowing agent(s), i.e. the reaction is irreversible.    -   (c) Mixed physical/chemical blowing agents: e.g. used to produce        flexible polyurethane (PU) foams with very low densities. Both        the chemical and physical blowing can be used in tandem to        balance each other out with respect to thermal energy        released/absorbed; hence, minimizing temperature rise. For        instance, isocyanate and water (which react to form CO₂) are        used in combination with liquid CO₂ (which boils to give gaseous        form) in the production of very low density flexible PU foams        for mattresses.    -   (d) Mechanically injected agents: Mechanically made foams        involve methods of introducing bubbles into liquid polymerizable        matrices (e.g. an unvulcanized elastomer in the form of a liquid        latex). Methods include whisking-in air or other gases or low        boiling volatile liquids in low viscosity lattices, or the        injection of a gas into an extruder barrel or a die, or into        injection molding barrels or nozzles and allowing the shear/mix        action of the screw to disperse the gas uniformly to form very        fine bubbles or a solution of gas in the melt. When the melt is        molded or extruded and the part is at atmospheric pressure, the        gas comes out of solution expanding the polymer melt immediately        before solidification.    -   (e) Soluble and leachable agents: Soluble fillers, e.g. solid        sodium chloride crystals mixed into a liquid urethane system,        which is then shaped into a solid polymer part, the sodium        chloride is later washed out by immersing the solid molded part        in water for some time, to leave small inter-connected holes in        relatively high density polymer products.    -   (f) We have found that the above five mechanisms can all be used        to create pores in the HA-derived materials while they are in a        solid state. Another mechanism of producing pores in a HA        material is through the generation and vaporization of volatile        gases by removing those non-carbon elements in a        high-temperature environment. This is a unique self-foaming        process that has never been previously taught or suggested.

The pore walls (cell walls) in the presently invented foam containchemically bonded and merged graphene-like hexagonal carbon atomicplanes. These planar aromatic molecules or hexagonal structured carbonatoms are well interconnected physically and chemically. The lateraldimensions (length or width) of these planes are huge (from 20 nm to >10μm), typically several times or even orders of magnitude larger than themaximum length/width of the starting humic acid molecules. The hexagonalcarbon atomic planes are essentially interconnected to form longelectron-conducting pathways with low resistance. This is a unique andnew class of material that has not been previously discovered,developed, or suggested to possibly exist.

In step (b), a HA suspension is formed into a wet layer on a solidsubstrate surface (e.g. PET film or glass) preferably under theinfluence of a shear stress. One example of such a shearing procedure iscasting or coating a thin film of HA suspension using a coating machine.This procedure is similar to a layer of varnish, paint, coating, or inkbeing coated onto a solid substrate. The roller, “doctor's blade”, orwiper can create a shear stress when the film is shaped, or when thereis a relative motion between the roller/blade/wiper and the supportingsubstrate at a sufficiently high relative motion speed. Quiteunexpectedly and significantly, such a shearing action enables theplanar HA molecules to well align along, for instance, the shearingdirection. Further surprisingly, such a molecular alignment state orpreferred orientation is not disrupted when the liquid components in theHA suspension are subsequently removed to form a well-packed layer ofhighly aligned sheet-like HA molecules that are at least partiallydried. The dried HA film has a high birefringence coefficient between anin-plane direction and the normal-to-plane direction.

In an embodiment, this HA layer is then subjected to a heat treatment toactivate the blowing agent and/or the thermally-induced reactions thatremove the non-carbon elements (e.g. F, O, etc.) from the HA moleculesto generate volatile gases as by-products. These volatile gases generatepores or bubbles inside the solid HA material, pushing sheet-like HAmolecules into a wall structure, forming a HA foam. If no blowing agentis added, the non-carbon elements in the HA material preferably occupyat least 10% by weight of the HA material (preferably at least 20%, andfurther preferably at least 30%). The first (initial) heat treatmenttemperature is typically greater than 80° C., preferably greater than100° C., more preferably greater than 300° C., further more preferablygreater than 500° C. and can be as high as 1,500° C. The blowing agentis typically activated at a temperature from 80° C. to 300° C., but canbe higher. The foaming procedure (formation of pores, cells, or bubbles)is typically completed within the temperature range of 80-1,500° C.Quite surprisingly, the chemical linking or merging between hexagonalcarbon atomic planes in an edge-to-edge and face-to-face manner (FIG. 2)can occur at a relatively low heat treatment temperature (e.g. as low asfrom 150 to 300° C.).

The HA-derived foam may be subjected to a further heat treatment thatinvolves at least a second temperature that is significantly higher thanthe first heat treatment temperature.

A properly programmed heat treatment procedure can involve just a singleheat treatment temperature (e.g. a first heat treatment temperatureonly), at least two heat treatment temperatures (first temperature for aperiod of time and then raised to a second temperature and maintained atthis second temperature for another period of time), or any othercombination of heat treatment temperatures (HTT) that involve an initialtreatment temperature (first temperature) and a final HTT (second),higher than the first. The highest or final HTT that the dried HA layerexperiences may be divided into four distinct HTT regimes:

-   -   Regime 1 (80° C. to 300° C.): In this temperature range (the        chemical linking and thermal reduction regime and also the        activation regime for a blowing agent, if present), HA layer        primarily undergoes thermally-induced chemical linking of        neighboring HA molecules, as schematically illustrated in the        upper portion of FIG. 2. This also involves removal of some        non-carbon atoms, such as 0 and H, leading to a reduction of        oxygen content from typically 20-42% (of O in HA) to        approximately 10-25%. This treatment results in a reduction of        inter-planar spacing in foam walls from approximately 0.6-1.2 nm        (as dried) down to approximately 0.4-0.6 nm, and an increase in        thermal conductivity to 100 W/mK per unit specific gravity        and/or electrical conductivity to 2,000 S/cm per unit of        specific gravity. (Since one can vary the level of porosity and,        hence, specific gravity of a graphene foam material and, given        the same graphene material, both the thermal conductivity and        electric conductivity values vary with the specific gravity,        these property values must be divided by the specific gravity to        facilitate a fair comparison.) Even with such a low temperature        range, some chemical linking between HA molecules occurs. The        inter-planar spacing remains relatively large (0.4 nm or        larger). Many O-containing functional groups survive (e.g. —OH        and —COOH).    -   Regime 2 (300° C.-1,500° C.): In this chemical linking and        merging regime, extensive chemical combination, polymerization,        and cross-linking between adjacent HA molecules or linked HA        molecules occur to form incipient graphene-like hexagonal carbon        atomic planes, as illustrated in lower portion of FIG. 2. The        oxygen content is reduced to typically from 2% to 10% (e.g.        after chemical linking and merging), resulting in a reduction of        inter-planar spacing to approximately 0.345 nm. This implies        that some initial graphitization has already begun at such a low        temperature, in stark contrast to conventional graphitizable        materials (such as carbonized polyimide film) that typically        require a temperature as high as 2,500° C. to initiate        graphitization. This is another distinct feature of the        presently invented graphene foam and its production processes.        These chemical linking reactions result in an increase in        thermal conductivity to >250 W/mK per unit of specific gravity,        and/or electrical conductivity to 2,500-4,000 S/cm per unit of        specific gravity.    -   Regime 3 (1,500-2,500° C.): In this ordering and graphitization        regime, extensive graphitization or merging of graphene-like        planes occurs, leading to significantly improved degree of        structural ordering in the foam walls. As a result, the oxygen        content is reduced to typically 0.1%-2% and the inter-graphene        spacing to approximately 0.337 nm (achieving degree of        graphitization from 1% to approximately 80%, depending upon the        actual HTT and length of time). The improved degree of ordering        is also reflected by an increase in thermal conductivity to >350        W/mK per unit of specific gravity, and/or electrical        conductivity to >3,500 S/cm per unit of specific gravity.    -   Regime 4 (higher than 2,500° C.): In this re-crystallization and        perfection regime, extensive movement and elimination of grain        boundaries and other defects occur, resulting in the formation        of nearly perfect single crystals or poly-crystalline        graphene-like crystals with huge grains in the foam walls, which        can be orders of magnitude larger than the original sizes of HA        molecules. The oxygen content is essentially eliminated,        typically 0% - 0.01%. The inter-planar spacing is reduced to        down to approximately 0.3354 nm (degree of graphitization from        80% to nearly 100%), corresponding to that of a perfect graphite        single crystal. The foamed structure thus obtained exhibits a        thermal conductivity of >400 W/mK per unit of specific gravity,        and electrical conductivity of >4,000 S/cm per unit of specific        gravity.

The presently invented HA-derived foam structure can be obtained byheat-treating the dried HA with a temperature program that covers atleast the first regime (typically requiring 1-4 hours in thistemperature range if the temperature never exceeds 500° C.), morecommonly covers the first two regimes (1-2 hours preferred), still morecommonly the first three regimes (preferably 0.5-2.0 hours in Regime 3),and can cover all the 4 regimes (including Regime 4 for 0.2 to 1 hour,may be implemented to achieve the highest conductivity).

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g +0.344 (1 - g), where d₀₀₂ is the interlayerspacing of graphite- or graphene-type crystal in nm. This equation isvalid only when d₀₀₂ is equal or less than approximately 0.3440 nm. TheHA-derived foam walls having a d₀₀₂ higher than 0.3440 nm reflects thepresence of oxygen-containing functional groups (such as —OH, >O, and—COOH on graphene-like molecular plane surfaces or edges) that act as aspacer to increase the inter-planar spacing.

Another structural index that can be used to characterize the degree ofordering of the stacked and bonded hexagonal carbon atomic planes in thefoam walls of HA-derived graphene-like and conventional graphitecrystals is the “mosaic spread,” which is expressed by the full width athalf maximum of a rocking curve (X-ray diffraction intensity) of the(002) or (004) reflection. This degree of ordering characterizes thegraphite or graphene crystal size (or grain size), amounts of grainboundaries and other defects, and the degree of preferred grainorientation. A nearly perfect single crystal of graphite ischaracterized by having a mosaic spread value of 0.2-0.4. Most of ourgraphene walls have a mosaic spread value in this range of 0.2-0.4 (ifproduced with a heat treatment temperature (HTT) no less than 2,500°C.). However, some values are in the range of 0.4-0.7 if the HTT isbetween 1,500 and 2,500° C., and in the range of 0.7-1.0 if the HTT isbetween 300 and 1,500° C.

Illustrated in FIG. 2 is a plausible chemical linking and mergingmechanism where only 2 aligned HA molecular segments are shown as anexample, although a large number of HA molecules can be chemicallylinked together and multiple “linked HA molecules) can be chemicallymerged to form a foam wall. Further, chemical linking could also occurface-to-face, not just edge-to-edge for HA molecules or sheets. Theselinking and merging reactions proceed in such a manner that themolecules are chemically merged, linked, and integrated into one singleentity. The resulting product is not a simple aggregate of individual HAsheets, but a single entity that is essentially a network ofinterconnected giant molecules with an essentially infinite molecularweight. All the constituent hexagonal carbon planes are very large inlateral dimensions (length and width) and, if the HTT is sufficientlyhigh (e.g. >1,500° C. or much higher), these planes are essentiallybonded together with one another.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the HA-derived foam walls are composed of several hugehexagonal carbon atomic planes (with length/width typically >>20 nm,more typically >>100 nm, often >>1 μm, and, in many cases, >>10 μm, oreven >>100 μm). These giant graphene-like planes are stacked and bondedalong the thickness direction (crystallographic c-axis direction) oftenthrough not just the van der Waals forces (as in conventional graphitecrystallites), but also covalent bonds, if the final heat treatmenttemperature is lower than 2,500° C. In these cases, wishing not to belimited by theory, but Raman and FTIR spectroscopy studies appear toindicate the co-existence of sp² (dominating) and sp³ (weak butexisting) electronic configurations, not just the conventional sp² ingraphite.

-   -   (1) This HA-derived graphitic foam wall is not made by gluing or        bonding discrete flakes/platelets together with a resin binder,        linker, or adhesive. Instead, HA molecules are merged through        joining or forming of covalent bonds with one another, into an        integrated graphene-like crystal entity, without using any        externally added linker or binder molecules or polymers.    -   (2) The foam wall is typically a poly-crystal composed of large        grains having incomplete grain boundaries. This entity is        derived from multiple HA molecules and these aromatic HA        molecules have lost their original identity. Upon removal of the        liquid component from the suspension, the resulting HA molecules        form an essentially amorphous structure. Upon heat treatments,        these HA molecules are chemically merged and linked into a        unitary or monolithic graphitic entity that constitutes the foam        wall. This foam wall is highly ordered.    -   (3) Due to these unique chemical composition (including oxygen        or non-carbon content), morphology, crystal structure (including        inter-planar spacing), and structural features (e.g. high degree        of orientations, few defects, incomplete grain boundaries,        chemical bonding and no gap between graphene sheets, and        substantially no interruptions in hexagonal carbon planes), the        HA-derived foam has a unique combination of outstanding thermal        conductivity, electrical conductivity, mechanical strength, and        stiffness (elastic modulus).

It may be further noted that a certain desired degree of hydrophilicitycan be imparted to the pore walls of the humic acid -derived foam if thenon-carbon element content (H and O) is from 2 to 20% by weight. Thesefeatures enable separation of oil from water by selectively absorbingoil from an oil-water mixture. In other words, such a HA-derived foammaterial is capable of recovering oil from water, helping to clean upoil-spilled river, lake, or ocean. The oil absorption capacity istypically from 50% to 500% of the foam's own weight. This is awonderfully useful material for environmental protection purposes.

If a high electrical or thermal conductivity is desired, the HA-carbonfoam can be subjected to graphitization treatment at a temperaturehigher than 2,500° C. The resulting material is particularly useful forthermal management applications (e.g. for use to make a finned heatsink, a heat exchanger, or a heat spreader).

It may be noted that the HA-carbon foam may be subjected to compressionduring and/or after the graphitization treatment. This operation enablesus to adjust the graphene sheet orientation and the degree of porosity.

In order to characterize the structure of the graphitic materialsproduced, X-ray diffraction patterns were obtained with an X-raydiffractometer equipped with CuKcv radiation. The shift and broadeningof diffraction peaks were calibrated using a silicon powder standard.The degree of graphitization, g, was calculated from the X-ray patternusing the Mering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), where d₀₀₂ is theinterlayer spacing of graphite or graphene crystal in nm. This equationis valid only when d₀₀₂ is equal or less than approximately 0.3440 nm.In the present study, the graphene-like (HA or RHA) foam walls having ad₀₀₂ higher than 0.3440 nm reflects the presence of oxygen-containingfunctional groups (such as —OH, >O, and —COOH on graphene molecularplane surfaces or edges) that act as a spacer to increase theinter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the stacked and bonded RHA planes in the foam walls ofgraphene and conventional graphite crystals is the “mosaic spread,”which is expressed by the full width at half maximum of a rocking curve(X-ray diffraction intensity) of the (002) or (004) reflection. Thisdegree of ordering characterizes the graphite or graphene crystal size(or grain size), amounts of grain boundaries and other defects, and thedegree of preferred grain orientation. A nearly perfect single crystalof graphite is characterized by having a mosaic spread value of 0.2-0.4.Most of our RHA walls have a mosaic spread value in this range of0.2-0.4 (if produced with a heat treatment temperature (HTT) no lessthan 2,500° C.). However, some values are in the range of 0.4-0.7 if theHTT is between 1,500 and 2,500° C., and in the range of 0.7-1.0 if theHTT is between 300 and 1,500° C.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the humic acid-carbon foam walls are composed of severallarge graphene planes (with length/width typically >>20 nm, moretypically >>100 nm, often >>1 μm, and, in many cases, >>10 μm). This isquite unexpected since the lateral dimensions (length and width) oforiginal humic acid sheets or molecules, prior to being heat treated,are typically <20 nm and more typically <10 nm. This implies that aplurality of HA sheets or molecules can be merged edge to edge throughcovalent bonds with one another, into a larger (longer or wider) sheet.

These large graphene-like planes also can be stacked and bonded alongthe thickness direction (crystallographic c-axis direction) oftenthrough not just the van der Waals forces (as in conventional graphitecrystallites), but also covalent bonds, if the final heat treatmenttemperature is lower than 2,500° C. In these cases, wishing not to belimited by theory, but Raman and FTIR spectroscopy studies appear toindicate the co-existence of sp² (dominating) and sp³ (weak butexisting) electronic configurations, not just the conventional sp² ingraphite.

The integral HA-derived foam is composed of multiple pores and porewalls, wherein the pore walls contain single-layer or few-layer HAsheets chemically bonded together, wherein the few-layer HA sheets have2-10 layers of stacked graphene-like merged HA planes having aninter-plane spacing d₀₀₂ from 0.3354 nm to 0.36 nm as measured by X-raydiffraction and the single-layer or few-layer graphene-like HA sheetscontain 0.01% to 25% by weight of non-carbon elements (more typically<15%).

The integral HA-derived foam typically has a density from 0.001 to 1.7g/cm³, a specific surface area from 50 to 3,000 m²/g, a thermalconductivity of at least 200 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 2,000 S/cm per unit of specificgravity. In a preferred embodiment, the pore walls contain stackedgraphene-like RHA planes having an inter-planar spacing d₀₀₂ from 0.3354nm to 0.40 nm as measured by X-ray diffraction.

Many of the HA sheets can be merged edge to edge through covalent bondswith one another, into an integrated reduced HA (RHA) entity. Due tothese unique chemical composition (including oxygen or hydrogen content,etc.), morphology, crystal structure (including inter-planar spacing),and structural features (e.g. degree of orientations, few defects,chemical bonding and no gap between graphene-like sheets, andsubstantially no interruptions along hexagonal plane directions), theHA-derived foam has a unique combination of outstanding thermalconductivity, electrical conductivity, mechanical strength, andstiffness (elastic modulus).

Thermal Management Applications

The aforementioned features and characteristics make the integralHA-derived foam an ideal element for a broad array of engineering andbiomedical applications. For instance, for thermal management purposesalone, the foam can be used in the following applications:

-   -   a) The HA-derived foam, being compressible and of high thermal        conductivity, is ideally suited for use as a thermal interface        material (TIM) that can be implemented between a heat source and        a heat spreader or between a heat source and a heat sink.    -   b) The HA-derived foam can be used as a heat spreader per se due        to its high thermal conductivity.    -   c) The HA-derived foam can be used as a heat sink or heat        dissipating material due to his high heat-spreading capability        (high thermal conductivity) and high heat-dissipating capability        (large number of surface pores inducing massive air-convection        micro or nano channels).    -   d) The light weight (low density adjustable between 0.001 and        1.8 g/cm³), high thermal conductivity per unit specific gravity        or per unit of physical density, and high structural integrity        (HA sheets being merged together to form long        electron-conducting paths) make this HA-derived foam an ideal        material for a durable heat exchanger.

The HA-derived foam foam-based thermal management or heat dissipatingdevices include a heat exchanger, a heat sink (e.g. finned heat sink), aheat pipe, high-conductivity insert, thin or thick conductive plate(between a heat sink and a heat source), thermal interface medium (orthermal interface material, TIM), thermoelectric or Peltier coolingplate, etc.

A heat exchanger is a device used to transfer heat between one or morefluids; e.g. a gas and a liquid separately flowing in differentchannels. The fluids are typically separated by a solid wall to preventmixing. The presently invented HA-derived foam material is an idealmaterial for such a wall provided the foam is not a totally open-cellfoam that allows for mixing of fluids. The presently invented methodenables production of both open-cell and closed-cell foam structures.The high surface pore areas enable dramatically faster exchange of heatsbetween the two or multiple fluids.

Heat exchangers are widely used in refrigeration systems, airconditioning units, heaters, power stations, chemical plants,petrochemical plants, petroleum refineries, natural-gas processing, andsewage treatment. A well-known example of a heat exchanger is found inan internal combustion engine in which a circulating engine coolantflows through radiator coils while air flows past the coils, which coolsthe coolant and heats the incoming air. The solid walls (e.g. thatconstitute the radiator coils) are normally made of a high thermalconductivity material, such as Cu and Al. The presently inventedHA-derived foam having either a higher thermal conductivity or higherspecific surface area is a superior alternative to Cu and Al, forinstance.

There are many types of heat exchangers that are commercially available:shell and tube heat exchanger, plate heat exchangers, plate and shellheat exchanger, adiabatic wheel heat exchanger, plate fin heatexchanger, pillow plate heat exchanger, fluid heat exchangers, wasteheat recovery units, dynamic scraped surface heat exchanger,phase-change heat exchangers, direct contact heat exchangers, andmicrochannel heat exchangers. Every one of these types of heatexchangers can take advantage of the exceptional high thermalconductivity and specific surface area of the presently invented foammaterial.

The presently invented solid HA-derived foam can also be used in a heatsink. Heat sinks are widely used in electronic devices for heatdissipation purposes. The central processing unit (CPU) and battery in aportable microelectronic device (such as a notebook computer, tablet,and smart phone) are well-known heat sources. Typically, a metal orgraphite object (e.g. Cu foil or graphite foil) is brought into contactwith the hot surface and this object helps to spread the heat to anexternal surface or outside air (primarily by conduction and convectionand to a lesser extent by radiation). In most cases, a thin thermalinterface material (TIM) mediates between the hot surface of the heatsource and a heat spreader or a heat-spreading surface of a heat sink.(The presently invented HA-derived foam can also be used as a TIM.)

A heat sink usually consists of a high-conductivity material structurewith one or more flat surfaces to ensure good thermal contact with thecomponents to be cooled, and an array of comb or fin like protrusions toincrease the surface contact with the air, and thus the rate of heatdissipation. A heat sink may be used in conjunction with a fan toincrease the rate of airflow over the heat sink. A heat sink can havemultiple fins (extended or protruded surfaces) to improve heat transfer.In electronic devices with limited amount of space, theshape/arrangement of fins must be optimized such that the heat transferdensity is maximized. Alternatively or additionally, cavities (invertedfins) may be embedded in the regions formed between adjacent fins. Thesecavities are effective in extracting heat from a variety of heatgenerating bodies to a heat sink.

Typically, an integrated heat sink comprises a heat collection member(core or base) and at least one heat dissipation member (e.g. a fin ormultiple fins) integral to the heat collection member (base) to form afinned heat sink. The fins and the core are naturally connected orintegrated together into a unified body without using an externallyapplied adhesive or mechanical fastening means to connect the fins tothe core. The heat collection base has a surface in thermal contact witha heat source (e.g. a LED), collects heat from this heat source, anddissipates heat through the fins into the air.

As illustrative examples, FIG. 10 provides a schematic of two heatsinks: 300 and 302. The first one contains a heat collection member (orbase member) 304 and multiple fins or heat dissipation members (e.g. fin306) connected to the base member 304. The base member 304 is shown tohave a heat collection surface 314 intended to be in thermal contactwith a heat source. The heat dissipation member or fin 306 is shown tohave at least a heat dissipation surface 320.

A particularly useful embodiment is an integrated radial heat sink 302comprising a radial finned heat sink assembly that comprises: (a) a base308 comprising a heat collection surface 318; and (b) a plurality ofspaced parallel planar fin members (e.g. 310, 312 as two examples)supported by or integral with the base 308, wherein the planar finmembers (e.g. 310) comprise the at least one heat dissipation surface322. Multiple parallel planar fin members are preferably equally spaced.

The presently invented HA-derived foam, being highly elastic andresilient, is itself a good thermal interface material and a highlyeffective heat spreading element as well. In addition, thishigh-conductivity foam can also be used as an inserts for electroniccooling and for enhancing the heat removal from small chips to a heatsink. Because the space occupied by high conductivity materials is amajor concern, it is a more efficient design to make use of highconductivity pathways that can be embedded into a heat generating body.The elastic and highly conducting solid graphene foam herein disclosedmeets these requirements perfectly.

The high elasticity and high thermal conductivity make the presentlyinvented solid HA-derived foam a good conductive thick plate to beplaced as a heat transfer interface between a heat source and a coldflowing fluid (or any other heat sink) to improve the coolingperformance. In such arrangement, the heat source is cooled under thethick HA-derived foam plate instead of being cooled in direct contactwith the cooling fluid. The thick plate of HA-derived foam cansignificantly improve the heat transfer between the heat source and thecooling fluid by way of conducting the heat current in an optimalmanner. No additional pumping power and no extra heat transfer surfacearea are required.

The HA-derived foam is also an outstanding material to construct a heatpipe. A heat pipe is a heat transfer device that uses evaporation andcondensation of a two-phase working fluid or coolant to transport largequantities of heat with a very small difference in temperature betweenthe hot and cold interfaces. A conventional heat pipe consists of sealedhollow tube made of a thermally conductive metal such as Cu or Al, and awick to return the working fluid from the evaporator to the condenser.The pipe contains both saturated liquid and vapor of a working fluid(such as water, methanol or ammonia), all other gases being excluded.However, both Cu and Al are prone to oxidation or corrosion and, hence,their performance degrades relatively fast over time. In contrast, theHA-derived foam is chemically inert and does not have these oxidation orcorrosion issues. The heat pipe for electronics thermal management canhave a foam envelope and wick, with water as the working fluid.HA-derived foam/methanol may be used if the heat pipe needs to operatebelow the freezing point of water, and HA-derived foam/ammonia heatpipes may be used for electronics cooling in space.

Peltier cooling plates operate on the Peltier effect to create a heatflux between the junction of two different conductors of electricity byapplying an electric current. This effect is commonly used for coolingelectronic components and small instruments. In practice, many suchjunctions may be arranged in series to increase the effect to the amountof heating or cooling required. The HA-derived foam may be used toimprove the heat transfer efficiency.

Filtration and Fluid Absorption Applications

The HA-derived foam can be made to contain microscopic pores (<2 nm) ormeso-scaled pores having a pore size from 2 nm to 50 nm. The HA-derivedfoam can also be made to contain micron-scaled pores (1-500 μm). Basedon well-controlled pore size alone, the instant HA-derived foam can bean exceptional filter material for air or water filtration.

Further, the humic acid (HA) pore wall chemistry can be controlled toimpart different amounts and/or types of functional groups to the porewalls (e.g. as reflected by the percentage of O, F, N, H, etc. in thefoam). In other words, the concurrent or independent control of bothpore sizes and chemical functional groups at different sites of theinternal structure provide unprecedented flexibility or highest degreeof freedom in designing and making HA-derived foams that exhibit manyunexpected properties, synergistic effects, and some unique combinationof properties that are normally considered mutually exclusive (e.g. somepart of the structure is hydrophobic and other part hydrophilic; or thefoam structure is both hydrophobic and oleophilic). A surface or amaterial is said to be hydrophobic if water is repelled from thismaterial or surface and that a droplet of water placed on a hydrophobicsurface or material will form a large contact angle. A surface or amaterial is said to be oleophilic if it has a strong affinity for oilsand not for water. The present method allows for precise control overhydrophobicity, hydrophilicity, and oleophilicity.

The present invention also provides an oil-removing, oil-separating, oroil-recovering device, which contains the presently invented HA-derivedfoam as an oil-absorbing or oil-separating element. Also provided is asolvent-removing or solvent-separating device containing the HA-derivedfoam as a solvent-absorbing element.

A major advantage of using the instant HA-derived foam as anoil-absorbing element is its structural integrity. Due to the notionthat HA sheets are chemically merged and thus of high structuralintegrity, the resulting foam would not get disintegrated upon repeatedoil absorption operations. In contrast, we have discovered thatgraphene-based oil-absorbing elements prepared by hydrothermalreduction, vacuum-assisted filtration, or freeze-drying getdisintegrated after absorbing oil for 2 or 3 times. There is justnothing (other than weak van der Waals forces existing prior to firstcontact with oil) to hold these otherwise separated graphene sheetstogether. Once these graphene sheets are wetted by oil, they no longerare able to return to the original shape of the oil-absorbing element.

Another major advantage of the instant technology is the flexibility indesigning and making oil-absorbing elements that are capable ofabsorbing oil up to an amount as high as 400 times of its own weight yetstill maintaining its structural shape (without significant expansion).This amount depends upon the specific pore volume of the foam, which canbe controlled mainly by the ratio between the amount of original carrierpolymer particles and the amount of HA molecules or sheets prior to theheat treatment.

The invention also provides a method to separate/recover oil from anoil-water mixture (e.g. oil-spilled water or waste water from oil sand).The method comprises the steps of (a) providing an oil-absorbing elementcomprising an integral HA-derived foam; (b) contacting an oil-watermixture with the element, which absorbs the oil from the mixture; and(c) retreating the oil-absorbing element from the mixture and extractingthe oil from the element. Preferably, the method comprises a furtherstep of (d) reusing the element.

Additionally, the invention provides a method to separate an organicsolvent from a solvent-water mixture or from a multiple-solvent mixture.The method comprises the steps of (a) providing an organicsolvent-absorbing element comprising an integral HA-derived foam; (b)bringing the element in contact with an organic solvent-water mixture ora multiple-solvent mixture containing a first solvent and at least asecond solvent; (c) allowing this element to absorb the organic solventfrom the mixture or absorb the first solvent from the at least secondsolvent; and (d) retreating the element from the mixture and extractingthe organic solvent or first solvent from the element. Preferably, themethod contains an additional step (e) of reusing the solvent-absorbingelement.

The following examples are used to illustrate some specific detailsabout the best modes of practicing the instant invention and should notbe construed as limiting the scope of the invention.

EXAMPLE 1 Humic Acid and Reduced Humic Acid from Leonardite

Humic acid can be extracted from leonardite by dispersing leonardite ina basic aqueous solution (pH of 10) with a very high yield (in the rangeof 75%). Subsequent acidification of the solution leads to precipitationof humic acid powder. In an experiment, 3 g of leonardite was dissolvedby 300 ml of double deionized water containing 1M KOH (or NH₄OH)solution under magnetic stirring. The pH value was adjusted to 10. Thesolution was then filtered to remove any big particles or any residualimpurities. The resulting humic acid dispersion, containing HC alone orwith the presence of a blowing agent, was cast onto a glass substrate toform a series of films for subsequent heat treatments.

In some samples, a chemical blowing agent (hydrazo dicarbonamide) wasadded to the suspension just prior to casting. The resulting suspensionwas then cast onto a glass surface using a doctor's blade to exert shearstresses, inducing HA molecular orientations. The resulting HA coatingfilms, after removal of liquid, have a thickness that can be varied fromapproximately 10 nm to 500 μm (preferably and typically from 1 μm to 50μm).

For making HA foam specimens, HA coating films were then subjected toheat treatments that typically involve an initial thermal reductiontemperature of 80-350° C. for 1-8 hours, followed by heat-treating at asecond temperature of 1,500-2,850° C. for 0.5 to 5 hours. It may benoted that we have found it essential to apply a compressive stress tothe coating film sample while being subjected to the first heattreatment. This compressing stress seems to have helped maintain goodcontacts between the HA molecules or sheets so that chemical merging andlinking between HA molecules or sheets can occur while pores are beingformed. Without such a compressive stress, the heat-treated filmtypically is excessively porous with constituent hexagonal carbon atomicplanes in the pore walls being very poorly oriented/positioned, andincapable of chemical merging and linking with one another. As a result,the thermal conductivity, electrical conductivity, and mechanicalstrength of the graphene foam are severely compromised.

EXAMPLE 2 Various Blowing Agents and Pore-forming (Bubble-producing)Processes

In the field of plastic processing, chemical blowing agents are mixedinto the plastic pellets in the form of powder or pellets and dissolvedat higher temperatures. Above a certain temperature specific for blowingagent dissolution, a gaseous reaction product (usually nitrogen or CO₂)is generated, which acts as a blowing agent. However, a chemical blowingagent cannot be dissolved in a graphene material, which is a solid, notliquid. This presents a challenge to make use of a chemical blowingagent to generate pores or cells in a graphene material.

After extensive experimenting, we have discovered that practically anychemical blowing agent (e.g. in a powder or pellet form) can be used tocreate pores or bubbles in a dried layer of graphene when the first heattreatment temperature is sufficient to activate the blowing reaction.The chemical blowing agent (powder or pellets) may be dispersed in theliquid medium to become a second component in the suspension, which canbe deposited onto the solid supporting substrate to form a wet layer.This wet layer of HA material may then be dried and heat treated toactivate the chemical blowing agent. After a chemical blowing agent isactivated and bubbles are generated, the resulting foamed HA structureis largely maintained even when subsequently a higher heat treatmenttemperature is applied to the structure. This is quite unexpected,indeed.

Chemical foaming agents (CFAs) can be organic or inorganic compoundsthat release gasses upon thermal decomposition. CFAs are typically usedto obtain medium- to high-density foams, and are often used inconjunction with physical blowing agents to obtain low-density foams.CFAs can be categorized as either endothermic or exothermic, whichrefers to the type of decomposition they undergo. Endothermic typesabsorb energy and typically release carbon dioxide and moisture upondecomposition, while the exothermic types release energy and usuallygenerate nitrogen when decomposed. The overall gas yield and pressure ofgas released by exothermic foaming agents is often higher than that ofendothermic types. Endothermic CFAs are generally known to decompose inthe range of 130 to 230° C. (266-446° F.), while some of the more commonexothermic foaming agents decompose around 200° C. (392° F.). However,the decomposition range of most exothermic CFAs can be reduced byaddition of certain compounds. The activation (decomposition)temperatures of CFAs fall into the range of our heat treatmenttemperatures. Examples of suitable chemical blowing agents includesodium bi-carbonate (baking soda), hydrazine, hydrazide,azodicarbonamide (exothermic chemical blowing agents), nitroso compounds(e.g. N,N-Dinitroso pentamethylene tetramine), hydrazine derivatives(e.g. 4.4′-Oxybis (benzenesulfonyl hydrazide) and Hydrazodicarbonamide), and hydrogen carbonate (e.g. Sodium hydrogen carbonate).These are all commercially available in plastics industry.

In the production of foamed plastics, physical blowing agents aremetered into the plastic melt during foam extrusion or injection moldedfoaming, or supplied to one of the precursor materials duringpolyurethane foaming. It has not been previously known that a physicalblowing agent can be used to create pores in a HA material, which is ina solid state (not melt). We have surprisingly observed that a physicalblowing agent (e.g. CO₂ or N₂) can be injected into the stream ofgraphene suspension prior to being coated or cast onto the supportingsubstrate. This would result in a foamed structure even when the liquidmedium (e.g. water and/or alcohol) is removed. The dried layer of HAmaterial is capable of maintaining a controlled amount of pores orbubbles during liquid removal and subsequent heat treatments.

Technically feasible blowing agents include Carbon dioxide (CO₂),Nitrogen (N₂), Isobutane (C₄H₁₀), Cyclopentane (C₅H₁₀), Isopentane(C₅H₁₂), CFC-11 (CFCI₃), HCFC-22 (CHF₂CI), HCFC-142b (CF₂CICH₃), andHCFC-134a (CH₂FCF₃). However, in selecting a blowing agent,environmental safety is a major factor to consider. The MontrealProtocol and its influence on consequential agreements pose a greatchallenge for the producers of foam. Despite the effective propertiesand easy handling of the formerly applied chlorofluorocarbons, there wasa worldwide agreement to ban these because of their ozone depletionpotential (ODP). Partially halogenated chlorofluorocarbons are also notenvironmentally safe and therefore already forbidden in many countries.The alternatives are hydrocarbons, such as isobutane and pentane, andthe gases such as CO₂ and nitrogen.

Except for those regulated substances, all the blowing agents recitedabove have been tested in our experiments. For both physical blowingagents and chemical blowing agents, the blowing agent amount introducedinto the suspension is defined as a blowing agent-to-HA material weightratio, which is typically from 0/1.0 to 1.0/1.0.

EXAMPLE 3 Preparation of Humic Acid from Coal

In a typical procedure, 300 mg of coal was suspended in concentratedsulfuric acid (60 ml) and nitric acid (20 ml), and followed by cupsonication for 2 h. The reaction was then stirred and heated in an oilbath at 100 or 120° C. for 24 h. The solution was cooled to roomtemperature and poured into a beaker containing 100 ml ice, followed bya step of adding NaOH (3M) until the pH value reached 7.

In one experiment, the neutral mixture was then filtered through a0.45-mm polytetrafluoroethylene membrane and the filtrate was dialyzedin 1,000 Da dialysis bag for 5 days. For the larger humic acid sheets,the time can be shortened to 1 to 2 h using cross-flow ultrafiltration.After purification, the solution was concentrated using rotaryevaporation to obtain solid humic acid sheets. These humic acid sheetsalone and their mixtures with a blowing agent were re-dispersed in asolvent (ethylene glycol and alcohol, separately) to obtain severaldispersion samples for subsequent casting or coating.

Various amounts (1%-30% by weight relative to HA material) of chemicalbowing agents (N,N-Dinitroso pentamethylene tetramine or 4.4′-Oxybis(benzenesulfonyl hydrazide) were added to a suspension containing HAsheets. The suspension was then cast onto a glass surface using adoctor's blade to exert shear stresses, inducing orientation and properpositioning of HA molecules or sheets. Several samples were cast,including one that was made using CO₂ as a physical blowing agentintroduced into the suspension just prior to casting. The resulting HAfilms, after removal of liquid, have a thickness that can be varied fromapproximately 1 to 100 μm.

The HA films were then subjected to heat treatments that involve aninitial (first) thermal reduction temperature of 80-1,500° C. for 1-5hours. This first heat treatment generated a HA foam (if HTT is <300°C.) and a foam of large sheet-like HA molecules or domains of hexagonalcarbon atomic planes in the pore walls (if HTT is from 300 to 1,500°C.). Some of the foam samples were then subjected to a secondtemperature of 1,500-2,850° C. to determine if the graphene-like domainsof hexagonal carbon atomic planes in the foam wall could be furtherperfected (graphitized to become more ordered or having a higher degreeof crystallinity).

COMPARATIVE EXAMPLE 3-a CVD Graphene Foams on Ni Foam Templates

The procedure was adapted from that disclosed in open literature: Chen,Z. et al. “Three-dimensional flexible and conductive interconnectedgraphene networks grown by chemical vapor deposition,” Nat. Mater. 10,424-428 (2011). Nickel foam, a porous structure with an interconnected3D scaffold of nickel was chosen as a template for the growth ofgraphene foam. Briefly, carbon was introduced into a nickel foam bydecomposing CH₄ at 1,000° C. under ambient pressure, and graphene filmswere then deposited on the surface of the nickel foam. Due to thedifference in the thermal expansion coefficients between nickel andgraphene, ripples and wrinkles were formed on the graphene films. Inorder to recover (separate) graphene foam, Ni frame must be etched away.Before etching away the nickel skeleton by a hot HCl (or FeCl₃)solution, a thin layer of poly(methyl methacrylate) (PMMA) was depositedon the surface of the graphene films as a support to prevent thegraphene network from collapsing during nickel etching. After the PMMAlayer was carefully removed by hot acetone, a fragile graphene foamsample was obtained. The use of the PMMA support layer is critical topreparing a free-standing film of graphene foam; only a severelydistorted and deformed graphene foam sample was obtained without thePMMA support layer. This is a tedious process that is notenvironmentally benign and is not scalable.

COMPARATIVE EXAMPLE 3-b Conventional Graphitic Foam from Pitch-basedCarbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 meso-phase pitchwas utilized. The sample is evacuated to less than 1 torr and thenheated to a temperature approximately 300° C. At this point, the vacuumwas released to a nitrogen blanket and then a pressure of up to 1,000psi was applied. The temperature of the system was then raised to 800°C. This was performed at a rate of 2 degree C/min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C/min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C. undera nitrogen blanket and then heat treated in separate runs in a graphitecrucible to 2500° C. and 2800° C. in Argon.

Samples from the foam were machined into specimens for measuring thethermal conductivity. The bulk thermal conductivity ranged from 67 W/mKto 151 W/mK. The density of the samples was from 0.31-0.61 g/cm³. Whenweight is taken into account, the specific thermal conductivity of thepitch derived foam is approximately 67/0.31=216 and 151/0.61=247.5 W/mKper specific gravity (or per physical density).

The compression strength of the samples having an average density of0.51 g/cm³ was measured to be 3.6 MPa and the compression modulus wasmeasured to be 74 MPa. By contrast, the compression strength andcompressive modulus of the presently invented HA-derived graphitic foamhaving a comparable physical density are 5.7 MPa and 103 MPa,respectively.

Shown in FIG. 3(A) are the thermal conductivity values vs. specificgravity of the HA-derived foam, meso-phase pitch-derived graphite foam,and Ni foam template-assisted CVD graphene foam. These data clearlydemonstrate the following unexpected results:

-   -   1) HA-derived foams produced by the presently invented process        exhibit significantly higher thermal conductivity as compared to        both meso-phase pitch-derived graphite foam and Ni foam        template-assisted CVD graphene, given the same physical density.    -   2) This is quite surprising in view of the notion that CVD        graphene is essentially pristine graphene that has never been        exposed to oxidation and should have exhibited a much higher        thermal conductivity compared to HA-derived hexagonal carbon        atomic planes, which are highly defective (having a high defect        population and, hence, low conductivity) after the        oxygen-containing functional groups are removed via conventional        thermal or chemical reduction methods. These exceptionally high        thermal conductivity values observed with the HA-derived        graphitic foams herein produced are much to our surprise.    -   3) Given the same amount of solid material, the presently        invented HA-derived foam after a heat treatment at a        HTT>1,500° C. is intrinsically most conducting, reflecting a        high level of graphitic crystal perfection (larger crystal        dimensions, fewer grain boundaries and other defects, better        crystal orientation, etc.). This is also unexpected.    -   4) The specific conductivity values of the presently invented        HA-derived foam and fluorinated HA-derived foams (FIG. 5)        exhibit values from 250 to 490 W/mK per unit of specific        gravity; but those of the other two foam materials are typically        lower than 250 W/mK per unit of specific gravity.

COMPARATIVE EXAMPLE 3-c Preparation of Pristine Graphene Foam (0%Oxygen)

Recognizing the possibility of the high defect population in HA sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to agraphene foam having a higher thermal conductivity. Pristine graphenesheets were produced by using the direct ultrasonication process (alsoknown as the liquid-phase exfoliation in the art).

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson S450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.There are no other non-carbon elements.

Various amounts (1%-30% by weight relative to graphene material) ofchemical bowing agents (N,N-Dinitroso pentamethylene tetramine or4.4′-Oxybis (benzenesulfonyl hydrazide) were added to a suspensioncontaining pristine graphene sheets and a surfactant. The suspension wasthen cast onto a glass surface. Several samples were cast, including onethat was made using CO₂ as a physical blowing agent introduced into thesuspension just prior to casting. The resulting graphene films, afterremoval of liquid, have a thickness that can be varied fromapproximately 10 to 100 μm. The graphene films were then subjected toheat treatments at a temperature of 80-1,500° C. for 1-5 hours, whichgenerated a graphene foam.

Summarized in FIG. 6 are thermal conductivity data for a series ofHA-derived foams and a series of pristine graphene derived foams, bothplotted over the same final (maximum) heat treatment temperatures. Thesedata indicate that the thermal conductivity of the HA-derived foams ishighly sensitive to the final heat treatment temperature (HTT). Evenwhen the HTT is very low, clearly some type of HA molecular linking andmerging or crystal perfection reactions have already been activated. Thethermal conductivity increases monotonically with the final HTT. Incontrast, the thermal conductivity of pristine graphene foams remainsrelatively constant until a final HTT of approximately 2,500° C. isreached, signaling the beginning of a re-crystallization and perfectionof graphite crystals. There are no functional groups in pristinegraphene, such as —COOH and —OH in HA, that enable chemical linking ofmolecules at relatively low HTTs. With a HTT as low as 1,250° C., HAmolecules and resulting hexagonal carbon atomic planes can merge to formsignificantly larger graphene-like hexagonal carbon sheets with reducedgrain boundaries and fewer electron transport path interruptions. Eventhough HA-derived sheets are intrinsically more defective than pristinegraphene, the presently invented process enables the HA molecules toform graphitic foams that outperform pristine graphene foams. This isanother unexpected result.

COMPARATIVE EXAMPLE 3-d: Preparation of Graphene Oxide (GO) Suspensionfrom Natural Graphite and Graphene Foams from Hydrothermally ReducedGraphene Oxide

Graphite oxide was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid for 48 hours, the suspension or slurryappears and remains optically opaque and dark. After 48 hours, thereacting mass was rinsed with water 3 times to adjust the pH value to atleast 3.0. A final amount of water was then added to prepare a series ofGO-water suspensions. We observed that GO sheets form a liquid crystalphase when GO sheets occupy a weight fraction >3% and typically from 5%to 15%.

A self-assembled graphene hydrogel (SGH) sample was then prepared by ahydrothermal method. In a typical procedure, the SGH can be easilyprepared by heating 2 mg/mL of homogeneous graphene oxide (GO) aqueousdispersion sealed in a Teflon-lined autoclave at 180° C. for 12 h. TheSGH containing about 2.6% (by weight) graphene sheets and 97.4% waterhas an electrical conductivity of approximately 5 x 10⁻³ S/cm. Upondrying and heat treating at 1,500° C., the resulting graphene foamexhibits an electrical conductivity of approximately 1.5×10⁻¹ S/cm,which is 2 times lower than those of the presently invented HA-derivedfoams produced by heat treating at the same temperature.

COMPARATIVE EXAMPLE 3-e Plastic Bead Template-assisted Formation ofReduced Graphene Oxide Foams

A hard template-directed ordered assembly for a macro-porous bubbledgraphene film (MGF) was prepared. Mono-disperse poly methyl methacrylate(PMMA) latex spheres were used as the hard templates. The GO liquidcrystal prepared in Comparative Example 3-d above was mixed with a PMMAspheres suspension. Subsequent vacuum filtration was then conducted toprepare the assembly of PMMA spheres and GO sheets, with GO sheetswrapped around the PMMA beads. A composite film was peeled off from thefilter, air dried and calcinated at 800° C. to remove the PMMA templateand thermally reduce GO into RGO simultaneously. The grey free-standingPMMA/GO film turned black after calcination, while the graphene filmremained porous.

FIG. 3(B) shows the thermal conductivity values of the presentlyinvented HA-derived foam, GO foam produced via sacrificial plastic beadtemplate-assisted process, and hydrothermally reduced GO graphene foam.Most surprisingly, given the same HTTs, the presently inventedHA-derived foam exhibits the highest thermal conductivity. Electricalconductivity data summarized in FIG. 4 are also consistent with thisconclusion. These data further support the notion that, given the sameamount of solid material, the presently invented HA suspensiondeposition (with stress-induced orientation) and subsequent heattreatments give rise to a HA-derived foam that is intrinsically mostconducting, reflecting a highest level of graphitic crystal perfection(larger crystal dimensions, fewer grain boundaries and other defects,better crystal orientation, etc. along the pore walls).

It is of significance to point out that all the prior art processes forproducing graphite foams or graphene foams appear to providemacro-porous foams having a physical density in the range ofapproximately 0.2- 0.6 g/cm³ only with pore sizes being typically toolarge (e.g. from 20 to 300 μm) for most of the intended applications. Incontrast, the instant invention provides processes that generateHA-derived foams having a density that can be as low as 0.01 g/cm³ andas high as 1.7 g/cm³. The pore sizes can be varied between meso-scaled(2-50 nm) up to macro-scaled (1-500 μm) depending upon the contents ofnon-carbon elements and the amount/type of blowing agent used. Thislevel of flexibility and versatility in designing various types ofgraphitic foams is unprecedented and un-matched by any prior artprocess.

EXAMPLES 4 Preparation of Fluorinated HA Foams

In a typical procedure, a sheet of HA-derived foam was fluorinated byvapors of chlorine trifluoride in a sealed autoclave reactor to yieldfluorinated HA-carbon hybrid film. Different durations of fluorinationtime were allowed for achieving different degrees of fluorination.Sheets of fluorinated HA-derived foam were then separately immersed incontainers each containing a chloroform-water mixture. We observed thatthese foam sheets selectively absorb chloroform from water and theamount of chloroform absorbed increases with the degree of fluorinationuntil the fluorine content reaches 7.2% by wt., as indicated in FIG. 9.

EXAMPLE 5 Preparation of Nitrogenated HA Foams

Several pieces of HA-derived foam prepared in Example 3 were immersed ina 30% H₂O₂-water solution for a period of 2-48 hours to obtain oxidizedHA-derived foams, having a controlled oxygen content of 2-25% by weight.

Some oxidized HA-derived foam samples were mixed with differentproportions of urea and the mixtures were heated in a microwave reactor(900 W) for 0.5 to 5 minutes. The products were washed several timeswith deionized water and vacuum dried. The products obtained werenitrogenated HA foam. The nitrogen contents were from 3% to 17.5 wt. %,as measured by elemental analysis.

It may be noted that different functionalization treatments of theHA-derived foam were for different purposes. For instance, oxidized HAfoam structures are particularly effective as an absorber of oil from anoil-water mixture (i.e. oil spilled on water and then mixed together),FIG. 7 and FIG. 8. In this case, the integral HA-derived foam structures(having 0-15% by wt. oxygen) are both hydrophobic and oleophilic (FIG.7). A surface or a material is said to be hydrophobic if water isrepelled from this material or surface and that a droplet of waterplaced on a hydrophobic surface or material will form a large contactangle. A surface or a material is said to be oleophilic if it has astrong affinity for oils and not for water.

Different contents of O, F, and/or N also enable the presently inventedHA-derived foams to absorb different organic solvents from water, or toseparate one organic solvent from a mixture of multiple solvents.

EXAMPLE 6 Characterization of Various Ha-derived Foams and ConventionalGraphite Foam

The internal structures (crystal structure and orientation) of severalseries of HA-carbon foam materials were investigated using X-raydiffraction. The X-ray diffraction curve of natural graphite typicallyexhibits a peak at approximately 2θ=26°, corresponds to aninter-graphene spacing (d₀₀₂) of approximately 0.3345 nm. The RHA wallsof the hybrid foam materials exhibit a d₀₀₂ spacing typically from0.3345 nm to 0.40 nm, but more typically up to 0.34 nm.

With a heat treatment temperature of 2,750° C. for the foam structureunder compression for one hour, the d₀₀₂ spacing is decreased toapproximately to 0.3354 nm, identical to that of a graphite singlecrystal. In addition, a second diffraction peak with a high intensityappears at 2θ=55° corresponding to X-ray diffraction from (004) plane.The (004) peak intensity relative to the (002) intensity on the samediffraction curve, or the I(004)/I(002) ratio, is a good indication ofthe degree of crystal perfection and preferred orientation ofgraphene-like planes. The (004) peak is either non-existing orrelatively weak, with the I(004)/I(002) ratio<0.1, for all graphiticmaterials heat treated at a temperature lower than 2,800° C. TheI(004)/I(002) ratio for the graphitic materials heat treated at3,000-3,250° C. (e.g., highly oriented pyrolytic graphite, HOPG) is inthe range of 0.2-0.5. In contrast, a graphene foam prepared with a finalHTT of 2,750° C. for one hour exhibits a I(004)/I(002) ratio of 0.78 anda Mosaic spread value of 0.21, indicating the pore walls being apractically perfect graphite single crystal with a good degree ofpreferred orientation (if prepared under a compression force).

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Some of our HA-derived foams have a mosaicspread value in this range of 0.3-0.6 when produced using a final heattreatment temperature no less than 2,500° C.

It is of significance to point out that a heat treatment temperature aslow as 500° C. is sufficient to bring the average inter-planar spacingbetween hexagonal carbon atomic planes along the pore walls to below 0.4nm, getting closer and closer to that of natural graphite or that of agraphite single crystal. The beauty of this approach is the notion thatthis HA suspension coating and heat treating strategy has enabled us toorganize, orient/align, and chemically merge the planar HA moleculesinto a unified structure with all the graphene-like hexagonal carbonatomic planes now being larger in lateral dimensions (significantlylarger than the length and width of the original HA molecules). Apotential chemical linking and merging mechanism is illustrated in FIG.3. This has given rise to exceptional thermal conductivity andelectrical conductivity values.

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of HA foam or HA-derivedgraphitic foam materials and related processes of production. Thechemical composition (% of oxygen, fluorine, and other non-carbonelements), structure (crystal perfection, grain size, defect population,etc), crystal orientation, morphology, process of production, andproperties of this new class of foam materials are fundamentallydifferent and patently distinct from meso-phase pitch-derived graphitefoam, CVD graphene-derived foam, and graphene foams from hydrothermalreduction of GO, and sacrificial bead template-assisted RGO foam. Thethermal conductivity, electrical conductivity, elastic modulus, andflexural strength exhibited by the presently invented foam materials aremuch higher than those of prior art foam materials.

We claim:
 1. A process for producing a humic acid-derived foam, saidprocess comprising: (a) preparing a humic acid dispersion havingmultiple humic acid molecules or sheets dispersed in a liquid medium,wherein said humic acid is selected from a group consisting offluorinated humic acid, chlorinated humic acid, brominated humic acid,iodized humic acid, and a combination thereof and wherein said humicacid has a content of non-carbon elements no less than 5% by weight andsaid dispersion contains a blowing agent having a blowing agent-to-humicacid weight ratio from 0/1.0 to 1.0/1.0; (b) dispensing and depositingsaid humic acid dispersion onto a surface of a supporting substrate toform a wet layer of humic acid; (c) partially or completely removingsaid liquid medium from the wet layer of humic acid to form a driedlayer of humic acid; and (d) heat treating the dried layer of humic acidat a first heat treatment temperature from 80° C. to 3,200° C. at adesired heating rate sufficient to induce volatile gas molecules fromsaid non-carbon elements or to activate said blowing agent for producingsaid humic acid-derived foam which is composed of multiple pores andpore walls and has a physical density from 0.005 to 1.7 g/cm³ and aspecific surface area from 50 to 3,200 m²/g, wherein said pore wallscontain single-layer or few-layer humic acid-derived hexagonal carbonatomic planes or sheets merged together in an edge-to-edge manner, saidfew-layer hexagonal carbon atomic planes or sheets have 2-10 layers ofstacked hexagonal carbon atomic planes having an inter-plane spacingd₀₀₂ from 0.3354 nm to 0.60 nm as measured by X-ray diffraction, andsaid single-layer or few-layer hexagonal carbon atomic planes contain0.01% to 25% by weight of non-carbon elements.
 2. The process of claim1, wherein said dispensing and depositing procedure includes subjectingsaid humic acid dispersion to an orientation-inducing stress.
 3. Theprocess of claim 1, further including a step of heat-treating the humicacid-derived foam at a second heat treatment temperature higher thansaid first heat treatment temperature for a length of time sufficientfor obtaining a graphitic foam wherein said pore walls contain stackedhexagonal carbon atomic planes having an inter-planar spacing d₀₀₂ from0.3354 nm to 0.36 nm and a content of non-carbon elements less than 2%by weight.
 4. The process of claim 1, wherein said blowing agent is aphysical blowing agent, a chemical blowing agent, a mixture thereof, adissolution-and-leaching agent, or a mechanically introduced blowingagent.
 5. The process of claim 1, which is a roll-to-roll processwherein said steps (b) and (c) include feeding said supporting substratefrom a feeder roller to a deposition zone, continuously orintermittently depositing said humic acid dispersion onto said surfaceof said supporting substrate to form said wet layer of humic acidthereon, drying said wet layer of humic acid to form the dried layer ofhumic acid, and collecting said dried layer of humic acid deposited onsaid supporting substrate on a collector roller.
 6. The process of claim1, wherein said first heat treatment temperature is from 100° C. to1,500° C.
 7. The process of claim 3, wherein said second heat treatmenttemperature includes at least a temperature selected from the groupconsisting of (A) 300-1,500° C., (B) 1,500-2,100° C., and (C)2,100-3,200° C.
 8. The process of claim 3, wherein said second heattreatment temperature includes a temperature in the range of 300-1,500°C. for at least 1 hour and then a temperature in the range of1,500-3,200° C. for at least 1 hour.
 9. The process of claim 3, whereinsaid non-carbon elements include an element selected from the groupconsisting of oxygen, fluorine, chlorine, bromine, iodine, nitrogen,hydrogen, and boron.
 10. The process of claim 3, wherein said a step ofheat treating the dried layer of humic acid at said first heat treatmenttemperature is conducted under a compressive stress.
 11. The process ofclaim 1, further comprising a compression step to reduce a thickness, apore size, or a porosity level of said foam.
 12. The process of claim 3,wherein said first and/or second heat treatment temperature contains atemperature in the range of 300° C.-1,500° C. and the foam has an oxygencontent or non-carbon content less than 1%, and pore walls having aninter-planar spacing less than 0.35 nm, a thermal conductivity of atleast 250 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 2,500 S/cm per unit of specific gravity. 13.The process of claim 3, wherein said first and/or second heat treatmenttemperature contains a temperature in the range of 1,500° C.-2,100° C.and the foam has an oxygen content or non-carbon content less than0.01%, pore walls having an inter-planar spacing less than 0.34 nm, athermal conductivity of at least 300 W/mK per unit of specific gravity,and/or an electrical conductivity no less than 3,000 S/cm per unit ofspecific gravity.
 14. The process of claim 3, wherein said first and/orsecond heat treatment temperature contains a temperature greater than2,100° C. and the foam has an oxygen content or non-carbon content nogreater than 0.001%, pore walls having an inter-planar spacing less than0.336 nm, a mosaic spread value no greater than 0.7, a thermalconductivity of at least 350 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 3,500 S/cm per unit of specificgravity.
 15. The process of claim 3, wherein said first and/or secondheat treatment temperature contains a temperature no less than 2,500° C.and the foam has pore walls containing stacked hexagonal carbon planeshaving an inter-planar spacing less than 0.336 nm, a mosaic spread valueno greater than 0.4, a thermal conductivity greater than 400 W/mK perunit of specific gravity, and/or an electrical conductivity greater than4,000 S/cm per unit of specific gravity.
 16. The process of claim 1,wherein said humic acid-derived foam has a density from 0.005 to 1.7g/cm³, a specific surface area from 50 to 3,200 m²/g, a thermalconductivity of at least 100 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 500 S/cm per unit of specificgravity.
 17. The process of claim 3, wherein said humic acid-derivedfoam has a density from 0.005 to 1.7 g/cm³, a specific surface area from50 to 3,200 m²/g, a thermal conductivity of at least 100 W/mK per unitof specific gravity, and/or an electrical conductivity no less than 500S/cm per unit of specific gravity.
 18. A process for producing afluorinated humic acid-derived foam, said process comprising: (a)preparing a humic acid dispersion having multiple fluorinated humic acidmolecules or sheets dispersed in a liquid medium, and wherein saiddispersion contains an optional blowing agent having a blowingagent-to-fluorinated humic acid weight ratio from 0/1.0 o 1.0/1.0; (b)dispensing and depositing said fluorinated humic acid dispersion onto asurface of a supporting substrate to form a wet layer of fluorinatedhumic acid; (c) partially or completely removing said liquid medium fromthe wet layer of humic acid to form a dried layer of fluorinated humicacid; and (d) heat treating the dried layer of fluorinated humic acid ata first heat treatment temperature from 80° C. to 3,200° C. at a desiredheating rate sufficient to induce volatile gas molecules from non-carbonelements or to activate said blowing agent for producing saidfluorinated humic acid-derived foam, which is composed of multiple poresand pore walls, wherein said pore walls contain single-layer orfew-layer fluorinated humic acid-derived hexagonal carbon atomic planesor sheets merged together in an edge-to-edge manner, said few-layerhexagonal carbon atomic planes or sheets have 2-10 layers of stackedhexagonal carbon atomic planes having an inter-plane spacing d₀₀₂ from0.3354 nm to 0.60 nm as measured by X-ray diffraction, and saidsingle-layer or few-layer hexagonal carbon atomic planes contain 0.01%to 25% by weight of non-carbon elements.